cpuset: use parse_cpulist for setting cpus.exclusive

Previous patches made parse_cpulist handle empty cpu mask input.
Now use this helper for exclusive cpus setting. Also, compute_trialcs_xcpus
can be called with empty cpus and handles it correctly.

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