memcg: fix race in file_mapped accounting

Presently, memcg's FILE_MAPPED accounting has following race with
move_account (happens at rmdir()).

    increment page->mapcount (rmap.c)
    mem_cgroup_update_file_mapped()           move_account()
					      lock_page_cgroup()
					      check page_mapped() if
					      page_mapped(page)>1 {
						FILE_MAPPED -1 from old memcg
						FILE_MAPPED +1 to old memcg
					      }
					      .....
					      overwrite pc->mem_cgroup
					      unlock_page_cgroup()
    lock_page_cgroup()
    FILE_MAPPED + 1 to pc->mem_cgroup
    unlock_page_cgroup()

Then,
	old memcg (-1 file mapped)
	new memcg (+2 file mapped)

This happens because move_account see page_mapped() which is not guarded
by lock_page_cgroup().  This patch adds FILE_MAPPED flag to page_cgroup
and move account information based on it.  Now, all checks are synchronous
with lock_page_cgroup().

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Reviewed-by: Balbir Singh <balbir@in.ibm.com>
Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Andrea Righi <arighi@develer.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>