memcg: remove redundant function calls

remove_list/add_list uses page_cgroup_zoneinfo() in it.

So, it's called twice before and after lock.

	mz = page_cgroup_zoneinfo();
	lock();
	mz = page_cgroup_zoneinfo();
	....
	unlock();

And address of mz never changes.

This is not good. This patch fixes this behavior.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>