[POWERPC] Fix idr locking in init_new_context

We always need to serialize accesses to mmu_context_idr.

I hit this bug when testing with a small number of mmu contexts.
Signed-off-by: Sonny Rao <sonny@burdell.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/mm/mmu_context_64.c

@@ -44,7 +44,9 @@ again:
 		return err;
 
 	if (index > MAX_CONTEXT) {
+		spin_lock(&mmu_context_lock);
 		idr_remove(&mmu_context_idr, index);
+		spin_unlock(&mmu_context_lock);
 		return -ENOMEM;
 	}