cgroup/cpuset: add callback_lock helper

To modify cpuset, both cpuset_mutex and callback_lock are needed. Add
helpers for cpuset-v1 to get callback_lock.

Signed-off-by: Chen Ridong <chenridong@huawei.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>