MAINTAINERS: add mm/page_cgroup.c into memcg subsystem

AFAICS mm/page_cgroup.c is for memcg subsystem, but it was directed only
to generic cgroup maintainers.  Fix it.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/MAINTAINERS b/MAINTAINERS
index 21a871c..d1a1465 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -4147,6 +4147,7 @@
 L:	linux-mm@kvack.org
 S:	Maintained
 F:	mm/memcontrol.c
+F:	mm/page_cgroup.c
 
 MEMORY TECHNOLOGY DEVICES (MTD)
 M:	David Woodhouse <dwmw2@infradead.org>