cgroup: remove cgroup->parent

cgroup->parent is redundant as cgroup->self.parent can also be used to
determine the parent cgroup and we're moving towards using
cgroup_subsys_states as the fundamental structural blocks.  This patch
introduces cgroup_parent() which follows cgroup->self.parent and
removes cgroup->parent.

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