cpuset: Allow v2 behavior in v1 cgroup

Cpuset v2 has some useful behaviors that are not present in v1 because
of backward compatibility concern. One of that is the restoration of
the original cpu and memory node mask after a hot removal and addition
event sequence.

This patch makes the cpuset controller to check the
CGRP_ROOT_CPUSET_V2_MODE flag and use the v2 behavior if it is set.

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