cgroup: fix memory leak in cgroup_rm_cftypes()

The memory allocated in cgroup_add_cftypes() should be freed. The
effect of this bug is we leak a bit memory everytime we unload
cfq-iosched module if blkio cgroup is enabled.
Signed-off-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/cgroup.c

@@ -2889,7 +2889,8 @@ int cgroup_rm_cftypes(struct cgroup_subsys *ss, struct cftype *cfts)
 
 	list_for_each_entry(set, &ss->cftsets, node) {
 		if (set->cfts == cfts) {
-			list_del_init(&set->node);
+			list_del(&set->node);
+			kfree(set);
 			cgroup_cfts_commit(ss, cfts, false);
 			return 0;
 		}