pkt_sched: fq: avoid hang when quantum 0

Configuring fq with quantum 0 hangs the system, presumably because of a
non-interruptible infinite loop. Either way quantum 0 does not make sense.

Reproduce with:
sudo tc qdisc add dev lo root fq quantum 0 initial_quantum 0
ping 127.0.0.1
Signed-off-by: Kenneth Klette Jonassen <kennetkl@ifi.uio.no>
Acked-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fq.c

@@ -670,8 +670,14 @@ static int fq_change(struct Qdisc *sch, struct nlattr *opt)
 	if (tb[TCA_FQ_FLOW_PLIMIT])
 		q->flow_plimit = nla_get_u32(tb[TCA_FQ_FLOW_PLIMIT]);
 
-	if (tb[TCA_FQ_QUANTUM])
-		q->quantum = nla_get_u32(tb[TCA_FQ_QUANTUM]);
+	if (tb[TCA_FQ_QUANTUM]) {
+		u32 quantum = nla_get_u32(tb[TCA_FQ_QUANTUM]);
+
+		if (quantum > 0)
+			q->quantum = quantum;
+		else
+			err = -EINVAL;
+	}
 
 	if (tb[TCA_FQ_INITIAL_QUANTUM])
 		q->initial_quantum = nla_get_u32(tb[TCA_FQ_INITIAL_QUANTUM]);