cls_cgroup: fix an oops when removing a cgroup

When removing a cgroup, an oops was triggered immediately. The cause
is wrong kfree() in cgrp_destroy().
Signed-off-by: NLi Zefan <lizf@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/cls_cgroup.c

@@ -46,7 +46,7 @@ static struct cgroup_subsys_state *cgrp_create(struct cgroup_subsys *ss,
 
 static void cgrp_destroy(struct cgroup_subsys *ss, struct cgroup *cgrp)
 {
-	kfree(ss);
+	kfree(net_cls_state(cgrp));
 }
 
 static u64 read_classid(struct cgroup *cgrp, struct cftype *cft)