memcg: fix numa scan information update to be triggered by memory event

commit 889976db ("memcg: reclaim memory from nodes in round-robin
order") adds an numa node round-robin for memcg.  But the information is
updated once per 10sec.

This patch changes the update trigger from jiffies to memcg's event count.
 After this patch, numa scan information will be updated when we see 1024
events of pagein/pageout under a memcg.

[akpm@linux-foundation.org: attempt to repair code layout]
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Ying Han <yinghan@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>