mm, slab: memcg_link the SLAB's kmem_cache

All the root caches are linked into slab_root_caches which was introduced by the commit 510ded33e075 ("slab: implement slab_root_caches list") but it missed to add the SLAB's kmem_cache.

While experimenting with opt-in/opt-out kmem accounting, I noticed system crashes due to NULL dereference inside cache_from_memcg_idx() while deferencing kmem_cache.memcg_params.memcg_caches. The upstream clean kernel will not see these crashes but SLAB should be consistent with SLUB which does linked its boot caches (kmem_cache_node and kmem_cache) into slab_root_caches.

---
 mm/slab.c | 1 +
 1 file changed, 1 insertion(+)

index 324446621b3e..9095c3945425 100644
@@ -1283,6 +1283,7 @@ void __init kmem_cache_init(void)
 				  nr_node_ids * sizeof(struct kmem_cache_node *),
 				  SLAB_HWCACHE_ALIGN, 0, 0);
 	list_add(&kmem_cache->list, &slab_caches);
+	memcg_link_cache(kmem_cache);
 	slab_state = PARTIAL;
 
 	/*

2.16.2.804.g6dcf76e118-goog