net_sched: sch_cake: Add drop reasons

Add three qdisc-specific drop reasons and use them in sch_cake:

 1) SKB_DROP_REASON_QDISC_OVERLIMIT
    Whenever the total queue limit for a qdisc instance is exceeded
    and a packet is dropped to make room.

 2) SKB_DROP_REASON_QDISC_CONGESTED
    Whenever a packet is dropped by the qdisc AQM algorithm because
    congestion is detected.

 3) SKB_DROP_REASON_CAKE_FLOOD
    Whenever a packet is dropped by the flood protection part of the
    CAKE AQM algorithm (BLUE).

Also use the existing SKB_DROP_REASON_QUEUE_PURGE in cake_clear_tin().

Reasons show up as:

perf record -a -e skb:kfree_skb sleep 1; perf script

          iperf3     665 [005]   848.656964: skb:kfree_skb: skbaddr=0xffff98168a333500 rx_sk=(nil) protocol=34525 location=__dev_queue_xmit+0x10f0 reason: QDISC_OVERLIMIT
         swapper       0 [001]   909.166055: skb:kfree_skb: skbaddr=0xffff98168280cee0 rx_sk=(nil) protocol=34525 location=cake_dequeue+0x5ef reason: QDISC_CONGESTED

Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>
Acked-by: Dave Taht <dave.taht@gmail.com>
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://patch.msgid.link/20241211-cake-drop-reason-v2-1-920afadf4d1b@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>