Use non atomic unlock

Slub can use the non-atomic version to unlock because other flags will not
get modified with the lock held.

Signed-off-by: Nick Piggin <npiggin@suse.de>
Acked-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/slub.c

@@ -1219,7 +1219,7 @@ static __always_inline void slab_lock(struct page *page)
 
 static __always_inline void slab_unlock(struct page *page)
 {
-	bit_spin_unlock(PG_locked, &page->flags);
+	__bit_spin_unlock(PG_locked, &page->flags);
 }
 
 static __always_inline int slab_trylock(struct page *page)