pkt_sched: fq: do not accept silly TCA_FQ_QUANTUM

As diagnosed by Florian :

If TCA_FQ_QUANTUM is set to 0x80000000, fq_deueue()
can loop forever in :

if (f->credit <= 0) {
  f->credit += q->quantum;
  goto begin;
}

... because f->credit is either 0 or -2147483648.

Let's limit TCA_FQ_QUANTUM to no more than 1 << 20 :
This max value should limit risks of breaking user setups
while fixing this bug.

Fixes: afe4fd0 ("pkt_sched: fq: Fair Queue packet scheduler")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Diagnosed-by: Florian Westphal <fw@strlen.de>
Reported-by: syzbot+dc9071cc5a85950bdfce@syzkaller.appspotmail.com
Signed-off-by: David S. Miller <davem@davemloft.net>

net/sched/sch_fq.c

@@ -786,10 +786,12 @@ static int fq_change(struct Qdisc *sch, struct nlattr *opt,
 	if (tb[TCA_FQ_QUANTUM]) {
 		u32 quantum = nla_get_u32(tb[TCA_FQ_QUANTUM]);
 
-		if (quantum > 0)
+		if (quantum > 0 && quantum <= (1 << 20)) {
 			q->quantum = quantum;
-		else
+		} else {
+			NL_SET_ERR_MSG_MOD(extack, "invalid quantum");
 			err = -EINVAL;
+		}
 	}
 
 	if (tb[TCA_FQ_INITIAL_QUANTUM])