memcg: simplify and inline __mem_cgroup_from_kmem

Before the previous patch ("memcg: unify slab and other kmem pages
charging"), __mem_cgroup_from_kmem had to handle two types of kmem - slab
pages and pages allocated with alloc_kmem_pages - memcg in the page
struct.  Now we can unify it.  Since after it, this function becomes tiny
we can fold it into mem_cgroup_from_kmem.

[hughd@google.com: move mem_cgroup_from_kmem into list_lru.c]
Signed-off-by: Vladimir Davydov <vdavydov@virtuozzo.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/list_lru.c

@@ -63,6 +63,16 @@ list_lru_from_memcg_idx(struct list_lru_node *nlru, int idx)
 	return &nlru->lru;
 }
 
+static __always_inline struct mem_cgroup *mem_cgroup_from_kmem(void *ptr)
+{
+	struct page *page;
+
+	if (!memcg_kmem_enabled())
+		return NULL;
+	page = virt_to_head_page(ptr);
+	return page->mem_cgroup;
+}
+
 static inline struct list_lru_one *
 list_lru_from_kmem(struct list_lru_node *nlru, void *ptr)
 {