cgroup/cpuset: Make child cpusets restrict parents on v1 hierarchy

The commit 1f1562fc ("cgroup/cpuset: Don't let child cpusets
restrict parent in default hierarchy") inteded to relax the check only
on the default hierarchy (or v2 mode) but it dropped the check in v1
too.

This patch returns and separates the legacy-only validations so that
they can be considered only in the v1 mode, which should enforce the old
constraints for the sake of compatibility.

Fixes: 1f1562fc

 ("cgroup/cpuset: Don't let child cpusets restrict parent in default hierarchy")
Suggested-by: Waiman Long <longman@redhat.com>
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>