cgroup/cpuset: Don't let child cpusets restrict parent in default hierarchy

In validate_change(), there is a check since v2.6.12 to make sure that
each of the child cpusets must be a subset of a parent cpuset.  IOW, it
allows child cpusets to restrict what changes can be made to a parent's
"cpuset.cpus". This actually violates one of the core principles of the
default hierarchy where a cgroup higher up in the hierarchy should be
able to change configuration however it sees fit as deligation breaks
down otherwise.

To address this issue, the check is now removed for the default hierarchy
to free parent cpusets from being restricted by child cpusets. The
check will still apply for legacy hierarchy.

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