cgroup: factor out cgroup_kn_lock_live() and cgroup_kn_unlock()

cgroup_mkdir(), cgroup_rmdir() and cgroup_subtree_control_write()
share the logic to break active protection so that they can grab
cgroup_tree_mutex which nests above active protection and/or remove
self.  Factor out this logic into cgroup_kn_lock_live() and
cgroup_kn_unlock().

This patch doesn't introduce any functional changes.

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