cpuset: remove cpuset->parent

cgroup already tracks the hierarchy.  Follow cgroup->parent to find
the parent and drop cpuset->parent.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Acked-by: Li Zefan <lizefan@huawei.com>