selftests/net: add csum offload test

Test NIC hardware checksum offload:

- Rx + Tx
- IPv4 + IPv6
- TCP + UDP

Optional features:

- zero checksum 0xFFFF
- checksum disable 0x0000
- transport encap headers
- randomization

See file header for detailed comments.

Expected results differ depending on NIC features:

- CHECKSUM_UNNECESSARY vs CHECKSUM_COMPLETE
- NETIF_F_HW_CSUM (csum_start/csum_off) vs NETIF_F_IP(V6)_CSUM
Signed-off-by: Willem de Bruijn <willemb@google.com>
Link: https://lore.kernel.org/r/20221128140210.553391-1-willemdebruijn.kernel@gmail.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/.gitignore

 # SPDX-License-Identifier: GPL-2.0-only
 bind_bhash
+csum
 cmsg_sender
 fin_ack_lat
 gro

tools/testing/selftests/net/Makefile

@@ -74,6 +74,7 @@ TEST_PROGS += test_ingress_egress_chaining.sh
 TEST_GEN_PROGS += so_incoming_cpu
 TEST_PROGS += sctp_vrf.sh
 TEST_GEN_FILES += sctp_hello
+TEST_GEN_FILES += csum
 
 TEST_FILES := settings