cgroup: fix obsolete function name above css_free_rwork_fn()

Since commit 8f36aaec9c ("cgroup: Use rcu_work instead of explicit rcu
and work item"), css_free_work_fn has been renamed to css_free_rwork_fn.
Update corresponding comment.

Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup/cgroup.c

@@ -5301,7 +5301,7 @@ static struct cftype cgroup_psi_files[] = {
  *    RCU callback.
  *
  * 4. After the grace period, the css can be freed.  Implemented in
- *    css_free_work_fn().
+ *    css_free_rwork_fn().
  *
  * It is actually hairier because both step 2 and 4 require process context
  * and thus involve punting to css->destroy_work adding two additional