memcg: do not open code accesses to res_counter members

We should use the accessor res_counter_read_u64 for that.

Although a purely cosmetic change is sometimes better delayed, to avoid
conflicting with other people's work, we are starting to have people
touching this code as well, and reproducing the open code behavior
because that's the standard =)

Time to fix it, then.

Signed-off-by: Glauber Costa <glommer@parallels.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -3763,7 +3763,7 @@ move_account:
 			goto try_to_free;
 		cond_resched();
 	/* "ret" should also be checked to ensure all lists are empty. */
-	} while (memcg->res.usage > 0 || ret);
+	} while (res_counter_read_u64(&memcg->res, RES_USAGE) > 0 || ret);
 out:
 	css_put(&memcg->css);
 	return ret;
@@ -3778,7 +3778,7 @@ try_to_free:
 	lru_add_drain_all();
 	/* try to free all pages in this cgroup */
 	shrink = 1;
-	while (nr_retries && memcg->res.usage > 0) {
+	while (nr_retries && res_counter_read_u64(&memcg->res, RES_USAGE) > 0) {
 		int progress;
 
 		if (signal_pending(current)) {