cgroup: Fix incorrect css_set_rwsem reference in comment

Since commit f0d9a5f1 ("cgroup: make css_set_rwsem a spinlock
and rename it to css_set_lock"), css_set_rwsem has been replaced by
css_set_lock. That commit, however, missed the css_set_rwsem reference
in include/linux/cgroup-defs.h. Fix that by changing it to css_set_lock
as well.
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

include/linux/cgroup-defs.h

@@ -238,7 +238,7 @@ struct css_set {
 	 * Lists running through all tasks using this cgroup group.
 	 * mg_tasks lists tasks which belong to this cset but are in the
 	 * process of being migrated out or in.  Protected by
-	 * css_set_rwsem, but, during migration, once tasks are moved to
+	 * css_set_lock, but, during migration, once tasks are moved to
 	 * mg_tasks, it can be read safely while holding cgroup_mutex.
 	 */
 	struct list_head tasks;