cpuset: use rebuild_sched_domains() in cpuset_hotplug_workfn()

In cpuset_hotplug_workfn(), partition_sched_domains() is called without
hotplug lock held, which is actually needed (stated in the function
header of partition_sched_domains()).

This patch tries to use rebuild_sched_domains() to solve the above
issue, and makes the code looks a little simpler.

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>