cgroup/cgroup.c: replace 'of->kn->priv' with of_cft()

we have supplied the inline function: of_cft() in cgroup.h.

So replace the direct use 'of->kn->priv' with inline func
of_cft(), which is more readable.
Signed-off-by: Hui Su <sh_def@163.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -3657,7 +3657,7 @@ static ssize_t cgroup_freeze_write(struct kernfs_open_file *of,
 
 static int cgroup_file_open(struct kernfs_open_file *of)
 {
-	struct cftype *cft = of->kn->priv;
+	struct cftype *cft = of_cft(of);
 
 	if (cft->open)
 		return cft->open(of);
@@ -3666,7 +3666,7 @@ static int cgroup_file_open(struct kernfs_open_file *of)
 
 static void cgroup_file_release(struct kernfs_open_file *of)
 {
-	struct cftype *cft = of->kn->priv;
+	struct cftype *cft = of_cft(of);
 
 	if (cft->release)
 		cft->release(of);
@@ -3677,7 +3677,7 @@ static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
 {
 	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
 	struct cgroup *cgrp = of->kn->parent->priv;
-	struct cftype *cft = of->kn->priv;
+	struct cftype *cft = of_cft(of);
 	struct cgroup_subsys_state *css;
 	int ret;
 
@@ -3727,7 +3727,7 @@ static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
 
 static __poll_t cgroup_file_poll(struct kernfs_open_file *of, poll_table *pt)
 {
-	struct cftype *cft = of->kn->priv;
+	struct cftype *cft = of_cft(of);
 
 	if (cft->poll)
 		return cft->poll(of, pt);