slub: fix ksize() for zero-sized pointers

The slab and slob allocators already did this right, but slub would call
"get_object_page()" on the magic ZERO_SIZE_PTR, with all kinds of nasty
end results.

Noted by Ingo Molnar.

Cc: Ingo Molnar <mingo@elte.hu>
Cc: Christoph Lameter <clameter@sgi.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slub.c

@@ -2394,7 +2394,7 @@ size_t ksize(const void *object)
 	struct page *page;
 	struct kmem_cache *s;
 
-	if (object == ZERO_SIZE_PTR)
+	if (ZERO_OR_NULL_PTR(object))
 		return 0;
 
 	page = get_object_page(object);