memcg: update maintainer list

Nishimura-san have been working for memcg very good.  His review and tests
give us much improvements and account migraiton which he is now
challenging is really important.

He is a stakeholder.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Balbir Singh <balbir@linux.vnet.ibm.com>
Acked-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

MAINTAINERS

@@ -3628,7 +3628,7 @@ F:	mm/
 
 MEMORY RESOURCE CONTROLLER
 M:	Balbir Singh <balbir@linux.vnet.ibm.com>
-M:	Pavel Emelyanov <xemul@openvz.org>
+M:	Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
 M:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
 L:	linux-mm@kvack.org
 S:	Maintained