mm: memcontrol: catch root bypass in move precharge

mm: memcontrol: catch root bypass in move precharge

When mem_cgroup_try_charge() returns -EINTR, it bypassed the charge to the
root memcg.  But move precharging does not catch this and treats this case
as if no charge had happened, thus leaking a charge against root.  Because
of an old optimization, the root memcg's res_counter is not actually
charged right now, but it's still an imbalance and subsequent patches will
charge the root memcg again.

Catch those bypasses to the root memcg and properly cancel them before
giving up the move.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
Acked-by: Michal Hocko <mhocko@suse.cz>
Cc: Hugh Dickins <hughd@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vladimir Davydov <vdavydov@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>