net_sched: sch_fq: fix incorrect behavior for small weights

fq_dequeue() has a complex logic to find packets in one of the 3 bands.

As Neal found out, it is possible that one band has a deficit smaller
than its weight. fq_dequeue() can return NULL while some packets are
elligible for immediate transmit.

In this case, more than one iteration is needed to refill pband->credit.

With default parameters (weights 589824 196608 65536) bug can trigger
if large BIG TCP packets are sent to the lowest priority band.

Bisected-by: John Sperbeck <jsperbeck@google.com>
Diagnosed-by: Neal Cardwell <ncardwell@google.com>
Fixes: 29f834a ("net_sched: sch_fq: add 3 bands and WRR scheduling")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Neal Cardwell <ncardwell@google.com>
Link: https://patch.msgid.link/20240824181901.953776-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

net/sched/sch_fq.c

@@ -663,7 +663,9 @@ static struct sk_buff *fq_dequeue(struct Qdisc *sch)
 			pband = &q->band_flows[q->band_nr];
 			pband->credit = min(pband->credit + pband->quantum,
 					    pband->quantum);
-			goto begin;
+			if (pband->credit > 0)
+				goto begin;
+			retry = 0;
 		}
 		if (q->time_next_delayed_flow != ~0ULL)
 			qdisc_watchdog_schedule_range_ns(&q->watchdog,