compat: Unlock qdisc root lock in case of failure when dumping stats.

Linux versions prior to 4.8 have a bug where the root qdisc lock is
not released in tc_fill_qdisc (in sch_api.c) in some failure cases.
This can cause a lockup when dumping cake stats from multiple
qdiscs, for example. Fix this by explicitly unlocking the qdisc
lock in the failure case.

sch_cake.c

@@ -3049,6 +3049,9 @@ static int cake_dump_stats(struct Qdisc *sch, struct gnet_dump *d)
 
 nla_put_failure:
 	nla_nest_cancel(d->skb, stats);
+#if LINUX_VERSION_CODE < KERNEL_VERSION(4, 8, 0)
+	sch_tree_unlock(sch);
+#endif
 	return -1;
 }