slab: warn on zero-length allocations

slub warns on this, and we're working on making kmalloc(0) return NULL.
Let's make slab warn as well so our testers detect such callers more
rapidly.
Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/slab.c

@@ -789,6 +789,7 @@ static inline struct kmem_cache *__find_general_cachep(size_t size,
 	 */
 	BUG_ON(malloc_sizes[INDEX_AC].cs_cachep == NULL);
 #endif
+	WARN_ON_ONCE(size == 0);
 	while (size > csizep->cs_size)
 		csizep++;