cpuset: Don't always flush cpuset_migrate_mm_wq in cpuset_write_resmask

cpuset: Don't always flush cpuset_migrate_mm_wq in cpuset_write_resmask

It is unnecessary to always wait for the flush operation of
cpuset_migrate_mm_wq to complete in cpuset_write_resmask, as modifying
cpuset.cpus or cpuset.exclusive does not trigger mm migrations. The
flush_workqueue can be executed only when cpuset.mems is modified.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>