selftests/bpf: add a new test to check the consumer update case

The subtest sends 33 packets at one time on purpose to see if xsk
exitting __xsk_generic_xmit() updates the global consumer of tx queue
when reaching the max loop (max_tx_budget, 32 by default). The number 33
can avoid xskq_cons_peek_desc() updates the consumer when it's about to
quit sending, to accurately check if the issue that the first patch
resolves remains. The new case will not check this issue in zero copy
mode.

Signed-off-by: Jason Xing <kernelxing@tencent.com>
Acked-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20250703141712.33190-3-kerneljasonxing@gmail.com


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