memcg: mark stat field of mem_cgroup struct as __percpu

It fixes a lot of sparse warnings.
Signed-off-by: Kirill A. Shutemov <kirill@shutemov.name>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -304,7 +304,7 @@ struct mem_cgroup {
 	/*
 	 * percpu counter.
 	 */
-	struct mem_cgroup_stat_cpu *stat;
+	struct mem_cgroup_stat_cpu __percpu *stat;
 	/*
 	 * used when a cpu is offlined or other synchronizations
 	 * See mem_cgroup_read_stat().