cgroup/cpuset: move relax_domain_level to cpuset-v1.c

Setting domain level is not supported at cpuset v2, so move corresponding
code into cpuset-v1.c.

The 'cpuset_write_s64' and 'cpuset_read_s64' are only used for setting
domain level, move them to cpuset-v1.c. Currently, expose to cpuset.c.
After cpuset legacy interface files are move to cpuset-v1.c, they can
be static. The 'rebuild_sched_domains_locked' is exposed to cpuset-v1.c.

The change from original code is that using 'cpuset_lock' and
'cpuset_unlock' functions to lock or unlock cpuset_mutex.

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