MIPS: Use inline function to access current thread pointer.

With LTE this fixes this issue:

  LDFINAL vmlinux.o
arch/mips/built-in.o (symbol from plugin): In function `sgimc_init':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
kernel/built-in.o (symbol from plugin): In function `get_task_mm':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
mm/built-in.o (symbol from plugin): In function `iov_iter_single_seg_count':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
fs/built-in.o (symbol from plugin): In function `finish_no_open':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
ipc/built-in.o (symbol from plugin): In function `ipc_init_ids':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
security/built-in.o (symbol from plugin): In function `key_schedule_gc':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
crypto/built-in.o (symbol from plugin): In function `crypto_find_alg':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
block/built-in.o (symbol from plugin): In function `elv_rb_find':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
argv_split.o (symbol from plugin): In function `argv_free':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
dec_and_lock.o (symbol from plugin): In function `_atomic_dec_and_lock':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
extable.o (symbol from plugin): In function `sort_extable':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
flex_proportions.o (symbol from plugin): In function `fprop_global_init':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
idr.o (symbol from plugin): In function `idr_for_each':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
is_single_threaded.o (symbol from plugin): In function `current_is_single_threaded':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
kobject.o (symbol from plugin): In function `kobject_get':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
kobject_uevent.o (symbol from plugin): In function `add_uevent_var':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
plist.o (symbol from plugin): In function `plist_add':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
radix-tree.o (symbol from plugin): In function `radix_tree_lookup_slot':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
ratelimit.o (symbol from plugin): In function `___ratelimit':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
rwsem-spinlock.o (symbol from plugin): In function `__init_rwsem':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
show_mem.o (symbol from plugin): In function `show_mem':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
timerqueue.o (symbol from plugin): In function `timerqueue_iterate_next':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
vsprintf.o (symbol from plugin): In function `simple_strtoull':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
delay.o (symbol from plugin): In function `__delay':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
mips-atomic.o (symbol from plugin): In function `arch_local_irq_disable':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
uncached.o (symbol from plugin): In function `run_uncached':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
lib/built-in.o (symbol from plugin): In function `_bcd2bin':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
arch/mips/lib/built-in.o (symbol from plugin): In function `ioread8':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
drivers/built-in.o (symbol from plugin): In function `fb_notifier_call_chain':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
net/built-in.o (symbol from plugin): In function `sock_from_file':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here
klist.o (symbol from plugin): In function `klist_init':
(.text+0x0): multiple definition of `$28'
init/built-in.o (symbol from plugin):(.text+0x0): first defined here

It also makes the code a little more readable, so let's merge it.
Signed-off-by: NRalf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/thread_info.h

@@ -58,8 +58,12 @@ struct thread_info {
 #define init_stack		(init_thread_union.stack)
 
 /* How to get the thread information struct from C.  */
-register struct thread_info *__current_thread_info __asm__("$28");
-#define current_thread_info()  __current_thread_info
+static inline struct thread_info *current_thread_info(void)
+{
+	register struct thread_info *__current_thread_info __asm__("$28");
+
+	return __current_thread_info;
+}
 
 #endif /* !__ASSEMBLY__ */