memcg: generic filestat update interface

This patch extracts the core logic from mem_cgroup_update_file_mapped() as
mem_cgroup_update_file_stat() and adds a wrapper.

As a planned future update, memory cgroup has to count dirty pages to
implement dirty_ratio/limit.  And more, the number of dirty pages is
required to kick flusher thread to start writeback.  (Now, no kick.)

This patch is preparation for it and makes other statistics implementation
clearer.  Just a clean up.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Balbir Singh <balbir@linux.vnet.ibm.com>
Reviewed-by: Greg Thelen <gthelen@google.com>
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>