cgroup: add css_parent()

Currently, controllers have to explicitly follow the cgroup hierarchy
to find the parent of a given css.  cgroup is moving towards using
cgroup_subsys_state as the main controller interface construct, so
let's provide a way to climb the hierarchy using just csses.

This patch implements css_parent() which, given a css, returns its
parent.  The function is guarnateed to valid non-NULL parent css as
long as the target css is not at the top of the hierarchy.

freezer, cpuset, cpu, cpuacct, hugetlb, memory, net_cls and devices
are converted to use css_parent() instead of accessing cgroup->parent
directly.

* __parent_ca() is dropped from cpuacct and its usage is replaced with
  parent_ca().  The only difference between the two was NULL test on
  cgroup->parent which is now embedded in css_parent() making the
  distinction moot.  Note that eventually a css->parent field will be
  added to css and the NULL check in css_parent() will go away.

This patch shouldn't cause any behavior differences.

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

net/sched/cls_cgroup.c

@@ -50,9 +50,11 @@ static struct cgroup_subsys_state *cgrp_css_alloc(struct cgroup *cgrp)
 
 static int cgrp_css_online(struct cgroup *cgrp)
 {
-	if (cgrp->parent)
-		cgrp_cls_state(cgrp)->classid =
-			cgrp_cls_state(cgrp->parent)->classid;
+	struct cgroup_cls_state *cs = cgrp_cls_state(cgrp);
+	struct cgroup_cls_state *parent = css_cls_state(css_parent(&cs->css));
+
+	if (parent)
+		cs->classid = parent->classid;
 	return 0;
 }