slab, kmemleak: stop calling kmemleak_erase() unconditionally

When the gotten object is NULL (probably due to ENOMEM), kmemleak_erase() is
unnecessary here, It just sets NULL to where already is NULL.  Add a condition.
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: J. R. Okajima <hooanon05@yahoo.co.jp>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>

mm/slab.c

@@ -3109,6 +3109,7 @@ static inline void *____cache_alloc(struct kmem_cache *cachep, gfp_t flags)
 	 * per-CPU caches is leaked, we need to make sure kmemleak doesn't
 	 * treat the array pointers as a reference to the object.
 	 */
+	if (objp)
 		kmemleak_erase(&ac->entry[ac->avail]);
 	return objp;
 }