cgroup/cpuset: Check for partition roots with overlapping CPUs

cgroup/cpuset: Check for partition roots with overlapping CPUs

With the previous commit that eliminates the overlapping partition
root corner cases in the hotplug code, the partition roots passed down
to generate_sched_domains() should not have overlapping CPUs. Enable
overlapping cpuset check for v2 and warn if that happens.

This patch also has the benefit of increasing test coverage of the new
Union-Find cpuset merging code to cgroup v2.

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