selftests: net: verify fq per-band packet limit

Commit 29f834aa326e ("net_sched: sch_fq: add 3 bands and WRR
scheduling") introduces multiple traffic bands, and per-band maximum
packet count.

Per-band limits ensures that packets in one class cannot fill the
entire qdisc and so cause DoS to the traffic in the other classes.

Verify this behavior:
  1. set the limit to 10 per band
  2. send 20 pkts on band A: verify that 10 are queued, 10 dropped
  3. send 20 pkts on band A: verify that  0 are queued, 20 dropped
  4. send 20 pkts on band B: verify that 10 are queued, 10 dropped

Packets must remain queued for a period to trigger this behavior.
Use SO_TXTIME to store packets for 100 msec.

The test reuses existing upstream test infra. The script is a fork of
cmsg_time.sh. The scripts call cmsg_sender.

The test extends cmsg_sender with two arguments:

* '-P' SO_PRIORITY
  There is a subtle difference between IPv4 and IPv6 stack behavior:
  PF_INET/IP_TOS        sets IP header bits and sk_priority
  PF_INET6/IPV6_TCLASS  sets IP header bits BUT NOT sk_priority

* '-n' num pkts
  Send multiple packets in quick succession.
  I first attempted a for loop in the script, but this is too slow in
  virtualized environments, causing flakiness as the 100ms timeout is
  reached and packets are dequeued.

Also do not wait for timestamps to be queued unless timestamps are
requested.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20231116203449.2627525-1-willemdebruijn.kernel@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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