cgroup/cpuset: Optimize isolated partition only generate_sched_domains() calls

If only isolated partitions are being created underneath the cgroup root,
there will only be one sched domain with top_cpuset.effective_cpus. We can
skip the unnecessary sched domains scanning code and save some cycles.

Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>