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: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/cls_cgroup.c b/net/sched/cls_cgroup.c
index 0d68b19..31d95b1 100644
--- a/net/sched/cls_cgroup.c
+++ b/net/sched/cls_cgroup.c
@@ -46,7 +46,7 @@
 
 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)