cgroup: remove cgroup->control_kn

Now that cgroup_subtree_control_write() has access to the associated
kernfs_open_file and thus the kernfs_node, there's no need to cache it
in cgroup->control_kn on creation.  Remove cgroup->control_kn and use
@of->kn directly.
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Li Zefan <lizefan@huawei.com>

include/linux/cgroup.h

@@ -177,7 +177,6 @@ struct cgroup {
 
 	struct cgroup *parent;		/* my parent */
 	struct kernfs_node *kn;		/* cgroup kernfs entry */
-	struct kernfs_node *control_kn;	/* kn for "cgroup.subtree_control" */
 	struct kernfs_node *populated_kn; /* kn for "cgroup.subtree_populated" */
 
 	/*

kernel/cgroup.c

@@ -2580,7 +2580,7 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 	 * active_ref protection.
 	 */
 	cgroup_get(cgrp);
-	kernfs_break_active_protection(cgrp->control_kn);
+	kernfs_break_active_protection(of->kn);
 
 	mutex_lock(&cgroup_tree_mutex);
 
@@ -2697,7 +2697,7 @@ static ssize_t cgroup_subtree_control_write(struct kernfs_open_file *of,
 out_unlock_tree:
 	mutex_unlock(&cgroup_tree_mutex);
 out_unbreak:
-	kernfs_unbreak_active_protection(cgrp->control_kn);
+	kernfs_unbreak_active_protection(of->kn);
 	cgroup_put(cgrp);
 	return ret ?: nbytes;
 
@@ -2887,9 +2887,7 @@ static int cgroup_add_file(struct cgroup *cgrp, struct cftype *cft)
 		return ret;
 	}
 
-	if (cft->seq_show == cgroup_subtree_control_show)
-		cgrp->control_kn = kn;
-	else if (cft->seq_show == cgroup_populated_show)
+	if (cft->seq_show == cgroup_populated_show)
 		cgrp->populated_kn = kn;
 	return 0;
 }