[breakertt]

This PR adds basic CI: on every push and pull request, build homa.ko and util/, then run the unit tests with and without --ipv4.

To get the tests running in CI, we also fix some existing problems in HomaModule:

• ./unit always exited 0, even when tests failed
• --ipv4 was ignored by the first test in a run
• two tests failed under --ipv4 (one test bug, one real GSO type bug)

This PR includes three commits:

1. Fix GSO type for IPv4 packets sent from IPv6 sockets
   • gso_type was picked from the socket family, but homa_route_xmit picks IPv4/IPv6 from the peer address
   • so an IPv6 socket replying to an IPv4 client sent IPv4 packets marked TCPV6
   • tcp4_gso_segment rejects these with TCP hijacking and software GSO (by code reading, not reproduced)
2. Fix the unit tests' exit status and --ipv4
   • ./unit returns the test result
   • --ipv4 also applies to the first test
   • homa_rpc_get_info__basics calls mock_set_ipv6
3. Introduce GitHub Actions CI
   • Linux 6.17.8 from kernel.org: defconfig, the options the mocks need, make modules_prepare
   • build homa.ko
   • build util/ (no tests there to run)
   • run the unit tests (IPv6 by default)
   • run the unit tests with --ipv4
   • about 1.5 minutes per run

Our Actions runs: https://github.com/breakertt/HomaModule/actions

• 39700840087, this PR: green, 863/863 in both modes

Notes:

• As of today, GitHub Actions quota is unlimited for public repos, so this costs nothing
• When HomaModule moves to another Linux version, LINUX_VERSION in .github/workflows/ci.yml needs to change with it

Future plans:

• when I have time, fix the unit tests of the strip version and add them to CI. Question for you: should the strip version always compile against the latest Linux release, net-next, or the latest rc?
• not included for now, as they would likely need resources beyond the free GitHub-hosted runners:
  • basic AI review on PRs
  • real kernel tests like netdev's CI, which boots a kernel and runs selftests on it