cpuset: use partition_cpus_change for setting exclusive cpus

A previous patch has introduced a new helper function
partition_cpus_change(). Now replace the exclusive cpus setting logic
with this helper function.

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