[SPARC64]: Bulletproof MMU context locking.

1) Always spin_lock_init() in init_context().  The caller essentially
   clears it out, or copies the mm info from the parent.  In both
   cases we need to explicitly initialize the spinlock.

2) Always do explicit IRQ disabling while taking mm->context.lock
   and ctx_alloc_lock.

Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc64/mm/tsb.c

@@ -354,6 +354,7 @@ void tsb_grow(struct mm_struct *mm, unsigned long rss, gfp_t gfp_flags)
 
 int init_new_context(struct task_struct *tsk, struct mm_struct *mm)
 {
+	spin_lock_init(&mm->context.lock);
 
 	mm->context.sparc64_ctx_val = 0UL;