[PATCH] slab: fix crash on __drain_alien_cahce() during CPU Hotplug

transfer_objects should only be called when all of the cpus in the
node are online.  CPU_DEAD notifier callback marks l3->shared to NULL.
Signed-off-by: NJacob Shin <jacob.shin@amd.com>
Signed-off-by: NLinus Torvalds <torvalds@osdl.org>

mm/slab.c

@@ -979,6 +979,7 @@ static void __drain_alien_cache(struct kmem_cache *cachep,
 		 * That way we could avoid the overhead of putting the objects
 		 * into the free lists and getting them back later.
 		 */
+		if (rl3->shared)
 			transfer_objects(rl3->shared, ac, ac->limit);
 
 		free_block(cachep, ac->entry, ac->avail, node);