net_sched: add qdisc_dequeue_drop() helper

net_sched: add qdisc_dequeue_drop() helper

Some qdisc like cake, codel, fq_codel might drop packets
in their dequeue() method.

This is currently problematic because dequeue() runs with
the qdisc spinlock held. Freeing skbs can be extremely expensive.

Add qdisc_dequeue_drop() method and a new TCQ_F_DEQUEUE_DROPS
so that these qdiscs can opt-in to defer the skb frees
after the socket spinlock is released.

TCQ_F_DEQUEUE_DROPS is an attempt to not penalize other qdiscs
with an extra cache line miss.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20251121083256.674562-14-edumazet@google.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>