mm/memcg: update page struct member in comments

The page->mem_cgroup member is replaced by memcg_data, and add a helper
page_memcg() for it.  Need to update comments to avoid confusing.

Link: https://lkml.kernel.org/r/1491c150-1cc0-6062-08ea-9c891548a3bc@linux.alibaba.comSigned-off-by: Alex Shi <alex.shi@linux.alibaba.com>
Acked-by: Roman Gushchin <guro@fb.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@kernel.org>
Cc: Vladimir Davydov <vdavydov.dev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -1324,7 +1324,7 @@ int mem_cgroup_scan_tasks(struct mem_cgroup *memcg,
  * @page: the page
  * @pgdat: pgdat of the page
  *
- * This function relies on page->mem_cgroup being stable - see the
+ * This function relies on page's memcg being stable - see the
  * access rules in commit_charge().
  */
 struct lruvec *mem_cgroup_page_lruvec(struct page *page, struct pglist_data *pgdat)
@@ -2879,7 +2879,7 @@ static void commit_charge(struct page *page, struct mem_cgroup *memcg)
 {
 	VM_BUG_ON_PAGE(page->mem_cgroup, page);
 	/*
-	 * Any of the following ensures page->mem_cgroup stability:
+	 * Any of the following ensures page's memcg stability:
 	 *
 	 * - the page lock
 	 * - LRU isolation