linux: add NLMSG_* helpers

[sbogomolov]

Description

Add the NLMSG_* message-framing helpers from linux/netlink.h. The length helpers take c_uint (the macros do unsigned arithmetic via NLMSG_ALIGNTO's 4U), while NLMSG_OK/NLMSG_NEXT take the signed byte count the walk loop gets from recv.

Sources

Linux NLMSG_* (linux/netlink.h):

• https://github.com/torvalds/linux/blob/4539944e515183668109bdf4d0c3d7d228383d88/include/uapi/linux/netlink.h#L98-L110

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are included
• Tested locally (cd libc-test && cargo test --target mytarget)

@rustbot label +stable-nominated

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[sbogomolov]

@tgross35 I think I've addressed all your comments.

@rustbot ready

[sbogomolov]

The docs-job failures are rustdoc OOM-ing on the ctest crate under nightly-2026-08-03, reproduces locally on clean main with cargo doc --workspace --no-deps (rustdoc SIGKILL). It is unrelated to this PR.

[tgross35]

Thanks!

View changes since this review

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.