Merge branch 'selftests/bpf: add coverage for xdp_features in test_progs'

Alexis Lothoré says:

====================
this small series aims to increase coverage of xdp features in
test_progs. The initial versions proposed to rework test_xdp_features.sh
to make it fit in test_progs, but some discussions in v1 and v2 showed
that the script is still needed as a standalone tool. So this new
revision lets test_xdp_features.sh as-is, and rather adds missing
coverage in existing test (cpu map). The new revision is now also a
follow-up to the update performed by Florian Kauer in [1] for devmap
programs testing.

[1] https://lore.kernel.org/bpf/20240911-devel-koalo-fix-ingress-ifindex-v4-2-5c643ae10258@linutronix.de/
---
Changes in v3:
- Drop xdp_features rework commit
- update xdp_cpumap_attach to extend its coverage
- Link to v2: https://lore.kernel.org/r/20240910-convert_xdp_tests-v2-1-a46367c9d038@bootlin.com

Changes in v2:
- fix endianness management in userspace packet parsing (call htonl on
  constant rather than packet part)

The new test has been run in a local x86 environment and in CI:
 #560/1   xdp_cpumap_attach/CPUMAP with programs in entries:OK
 #560/2   xdp_cpumap_attach/CPUMAP with frags programs in entries:OK
 #560     xdp_cpumap_attach:OK
 Summary: 1/2 PASSED, 0 SKIPPED, 0 FAILED
====================

Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>