selftests/net: Add netkit io_uring ZC test for large rx_buf_len

selftests/net: Add netkit io_uring ZC test for large rx_buf_len

Add test_iou_zcrx_large_buf, which runs iou-zcrx with rx_buf_len >
page size (-x 2) through a netkit-leased RX queue. The netkit ifindex
is opaque to io_uring, but rx_page_size is honoured by the leased
physical qops via netif_mp_open_rxq()'s lease redirect.

Originally, I also added a BIG TCP variant on top, but dropped it
here as fbnic (and the QEMU fbnic model) has no BIG TCP support
to exercise it as this point.

Tested against the QEMU fbnic emulation. The new test exercises
the > page rx_buf_len path only when the leased NIC advertises
QCFG_RX_PAGE_SIZE; otherwise it skips.

For fbnic, I used Bjorn's patches locally [0]:

  # ./nk_qlease.py
  TAP version 13
  1..5
  ok 1 nk_qlease.test_iou_zcrx
  ok 2 nk_qlease.test_iou_zcrx_large_buf
  ok 3 nk_qlease.test_attrs
  ok 4 nk_qlease.test_attach_xdp_with_mp
  ok 5 nk_qlease.test_destroy
  # Totals: pass:5 fail:0 xfail:0 xpass:0 skip:0 error:0

Without those patches (aka not advertising QCFG_RX_PAGE_SIZE):

  # ./nk_qlease.py
  TAP version 13
  1..5
  ok 1 nk_qlease.test_iou_zcrx
  ok 2 nk_qlease.test_iou_zcrx_large_buf # SKIP Large chunks are not supported -95
  ok 3 nk_qlease.test_attrs
  ok 4 nk_qlease.test_attach_xdp_with_mp
  ok 5 nk_qlease.test_destroy
  # Totals: pass:4 fail:0 xfail:0 xpass:0 skip:1 error:0

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Bobby Eshleman <bobbyeshleman@meta.com>
Link: https://lore.kernel.org/netdev/20260522113225.241337-1-bjorn@kernel.org/
Link: https://patch.msgid.link/20260614102607.863838-4-daniel@iogearbox.net
Signed-off-by: Jakub Kicinski <kuba@kernel.org>