mm: memcg: group swapped-out statistics counter logically

mm: memcg: group swapped-out statistics counter logically

The counter of currently swapped out pages in a memcg (hierarchy) is
sitting amidst ever-increasing event counters.  Move this item to the
other counters that reflect current state rather than history.

This technically breaks the kernel ABI, but hopefully nobody relies on the
order of items in memory.stat.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Ying Han <yinghan@google.com>
Cc: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>