MIPS: Init new mmu_context for each possible CPU to avoid memory corruption

Currently, init_new_context() only for each online CPU, this may cause
memory corruption when CPU hotplug and fork() happens at the same time.
To avoid this, we make init_new_context() cover each possible CPU.

Scenario:
1, CPU#1 is being offline;
2, On CPU#0, do_fork() call dup_mm() and copy a mm_struct to the child;
3, On CPU#0, dup_mm() call init_new_context(), since CPU#1 is offline
   and init_new_context() only covers the online CPUs, child has the
   same asid as its parent on CPU#1 (however, child's asid should be 0);
4, CPU#1 is being online;
5, Now, if both parent and child run on CPU#1, memory corruption (e.g.
   segfault, bus error, etc.) will occur.
Signed-off-by: NHuacai Chen <chenhc@lemote.com>
Acked-by: NDavid Daney <david.daney@cavium.com>
Patchwork: http://patchwork.linux-mips.org/patch/4995/Acked-by: NJohn Crispin <blogic@openwrt.org>

arch/mips/include/asm/mmu_context.h

@@ -133,7 +133,7 @@ init_new_context(struct task_struct *tsk, struct mm_struct *mm)
 {
 	int i;
 
-	for_each_online_cpu(i)
+	for_each_possible_cpu(i)
 		cpu_context(i, mm) = 0;
 
 	return 0;