cgroup/freezer: Add cgroup CGRP_FROZEN flag update helper

Add help to update cgroup CGRP_FROZEN flag. Both cgroup_propagate_frozen
and cgroup_update_frozen functions update CGRP_FROZEN flag, this makes
code concise.

Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>