memcg: simplify move_account() check

In memcg, for avoiding take-lock-irq-off at accessing page_cgroup, a
logic, flag + rcu_read_lock(), is used.  This works as following

     CPU-A                     CPU-B
                             rcu_read_lock()
    set flag
                             if(flag is set)
                                   take heavy lock
                             do job.
    synchronize_rcu()        rcu_read_unlock()
    take heavy lock.

In recent discussion, it's argued that using per-cpu value for this flag
just complicates the code because 'set flag' is very rare.

This patch changes 'flag' implementation from percpu to atomic_t.  This
will be much simpler.

Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Acked-by: Greg Thelen <gthelen@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Ying Han <yinghan@google.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>