mm/vmscan: replace zone_nr_lru_pages() with get_lruvec_size()

If memory cgroup is enabled we always use lruvecs which are embedded into
struct mem_cgroup_per_zone, so we can reach lru_size counters via
container_of().

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 39b2c14..a68db29 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -723,6 +723,15 @@
 }
 
 unsigned long
+mem_cgroup_get_lruvec_size(struct lruvec *lruvec, enum lru_list lru)
+{
+	struct mem_cgroup_per_zone *mz;
+
+	mz = container_of(lruvec, struct mem_cgroup_per_zone, lruvec);
+	return mz->lru_size[lru];
+}
+
+static unsigned long
 mem_cgroup_zone_nr_lru_pages(struct mem_cgroup *memcg, int nid, int zid,
 			unsigned int lru_mask)
 {