net_sched: fix a crash in tc_new_tfilter()

When tcf_block_find() fails, it already rollbacks the qdisc refcnt,
so its caller doesn't need to clean up this again. Avoid calling
qdisc_put() again by resetting qdisc to NULL for callers.

Reported-by: syzbot+37b8770e6d5a8220a039@syzkaller.appspotmail.com
Fixes: e368fdb6 ("net: sched: use Qdisc rcu API instead of relying on rtnl lock")
Signed-off-by: NCong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/sched/cls_api.c

@@ -717,8 +717,10 @@ static struct tcf_block *tcf_block_find(struct net *net, struct Qdisc **q,
 errout_rcu:
 	rcu_read_unlock();
 errout_qdisc:
-	if (*q)
+	if (*q) {
 		qdisc_put(*q);
+		*q = NULL;
+	}
 	return ERR_PTR(err);
 }