memcg: simplify unreclaimable groups handling in soft limit reclaim

If we fail to reclaim anything from a cgroup during a soft reclaim pass
we want to get the next largest cgroup exceeding its soft limit. To
achieve this, we should obviously remove the current group from the tree
and then pick the largest group. Currently we have a weird loop instead.
Let's simplify it.

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