sparc64: Make atomic_xchg() an inline function rather than a macro.

This avoids a lot of -Wunused warnings such as:

====================
kernel/debug/debug_core.c: In function ‘kgdb_cpu_enter’:
./arch/sparc/include/asm/cmpxchg_64.h:55:22: warning: value computed is not used [-Wunused-value]
 #define xchg(ptr,x) ((__typeof__(*(ptr)))__xchg((unsigned long)(x),(ptr),sizeof(*(ptr))))

./arch/sparc/include/asm/atomic_64.h:86:30: note: in expansion of macro ‘xchg’
 #define atomic_xchg(v, new) (xchg(&((v)->counter), new))
                              ^~~~
kernel/debug/debug_core.c:508:4: note: in expansion of macro ‘atomic_xchg’
    atomic_xchg(&kgdb_active, cpu);
    ^~~~~~~~~~~
====================
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

arch/sparc/include/asm/atomic_64.h

@@ -83,7 +83,11 @@ ATOMIC_OPS(xor)
 #define atomic64_add_negative(i, v) (atomic64_add_return(i, v) < 0)
 
 #define atomic_cmpxchg(v, o, n) (cmpxchg(&((v)->counter), (o), (n)))
-#define atomic_xchg(v, new) (xchg(&((v)->counter), new))
+
+static inline int atomic_xchg(atomic_t *v, int new)
+{
+	return xchg(&v->counter, new);
+}
 
 static inline int __atomic_add_unless(atomic_t *v, int a, int u)
 {