memcg: remove mem_cgroup_reclaimable check from soft reclaim

mem_cgroup_reclaimable() checks whether a cgroup has reclaimable pages on
*any* NUMA node.  However, the only place where it's called is
mem_cgroup_soft_reclaim(), which tries to reclaim memory from a *specific*
zone.  So the way it is used is incorrect - it will return true even if
the cgroup doesn't have pages on the zone we're scanning.

I think we can get rid of this check completely, because
mem_cgroup_shrink_node_zone(), which is called by
mem_cgroup_soft_reclaim() if mem_cgroup_reclaimable() returns true, is
equivalent to shrink_lruvec(), which exits almost immediately if the
lruvec passed to it is empty.  So there's no need to optimize anything
here.  Besides, we don't have such a check in the general scan path
(shrink_zone) either.

Signed-off-by: Vladimir Davydov <vdavydov@parallels.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>