memcg, vmscan: do not attempt soft limit reclaim if it would not scan anything

memcg, vmscan: do not attempt soft limit reclaim if it would not scan anything

mem_cgroup_should_soft_reclaim controls whether soft reclaim pass is done
and it always says yes currently.  Memcg iterators are clever to skip
nodes that are not soft reclaimable quite efficiently but
mem_cgroup_should_soft_reclaim can be more clever and do not start the
soft reclaim pass at all if it knows that nothing would be scanned anyway.

In order to do that, simply reuse mem_cgroup_soft_reclaim_eligible for the
target group of the reclaim and allow the pass only if the whole subtree
wouldn't be skipped.

Signed-off-by: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Glauber Costa <glommer@openvz.org>
Cc: Greg Thelen <gthelen@google.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Michel Lespinasse <walken@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Ying Han <yinghan@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>