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 <[e-mail]>
Cc: Christoph Lameter <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/slub.c

···
 	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);