memcg: fix reclaimable lru check in memcg

Now, in mem_cgroup_hierarchical_reclaim(), mem_cgroup_local_usage() is
used for checking whether the memcg contains reclaimable pages or not.  If
no pages in it, the routine skips it.

But, mem_cgroup_local_usage() contains Unevictable pages and cannot handle
"noswap" condition correctly.  This doesn't work on a swapless system.

This patch adds test_mem_cgroup_reclaimable() and replaces
mem_cgroup_local_usage().  test_mem_cgroup_reclaimable() see LRU counter
and returns correct answer to the caller.  And this new function has
"noswap" argument and can see only FILE LRU if necessary.

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: fix kerneldoc 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>