net/sched: Add precise drop reason for pfifo_fast queue overflows

Currently, packets dropped by pfifo_fast due to queue overflow are
marked with a generic SKB_DROP_REASON_QDISC_DROP in __dev_xmit_skb().

This patch adds explicit drop reason SKB_DROP_REASON_QDISC_OVERLIMIT
for queue-full cases, providing better distinction from other qdisc drops.

Signed-off-by: Fan Yu <fan.yu9@zte.com.cn>
Reviewed-by: Cong Wang <xiyou.wangcong@gmail.com>
Link: https://patch.msgid.link/20250724212837119BP9HOs0ibXDRWgsXMMir7@zte.com.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: Fan Yu

net/sched/sch_generic.c

@@ -740,6 +740,8 @@ static int pfifo_fast_enqueue(struct sk_buff *skb, struct Qdisc *qdisc,
 	err = skb_array_produce(q, skb);
 
 	if (unlikely(err)) {
+		tcf_set_drop_reason(skb, SKB_DROP_REASON_QDISC_OVERLIMIT);
+
 		if (qdisc_is_percpu_stats(qdisc))
 			return qdisc_drop_cpu(skb, qdisc, to_free);
 		else