cgroup: implement cgroup_is_descendant()

A couple controllers want to determine whether two cgroups are in
ancestor/descendant relationship.  As it's more likely that the
descendant is the primary subject of interest and there are other
operations focusing on the descendants, let's ask is_descendent rather
than is_ancestor.

Implementation is trivial as the previous patch guarantees that all
ancestors of a cgroup stay accessible as long as the cgroup is
accessible.

tj: Removed depth optimization, renamed from cgroup_is_ancestor(),
    rewrote descriptions.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>