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

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>