cgroup: add cgroup_subsys->css_released()

Add a new cgroup subsys callback css_released().  This is called when
the reference count of the css (cgroup_subsys_state) reaches zero
before RCU scheduling free.

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

kernel/cgroup.c

@@ -4380,6 +4380,8 @@ static void css_release_work_fn(struct work_struct *work)
 	if (ss) {
 		/* css release path */
 		cgroup_idr_remove(&ss->css_idr, css->id);
+		if (ss->css_released)
+			ss->css_released(css);
 	} else {
 		/* cgroup release path */
 		cgroup_idr_remove(&cgrp->root->cgroup_idr, cgrp->id);