Add Linux 6.4/6.7 write-protect features and Uffd::copy_with_mode

[bchalios]

Summary

This adds the two userfaultfd write-protect features introduced in Linux 6.4 and 6.7, and a way to request UFFDIO_COPY_MODE_WP when resolving a missing fault.

• linux6_4 feature: UFFD_FEATURE_WP_UNPOPULATED, exposed as FeatureFlags::WP_UNPOPULATED. Lets UFFDIO_WRITEPROTECT apply to pages that have never been populated.
• linux6_7 feature: UFFD_FEATURE_WP_ASYNC, exposed as FeatureFlags::WP_ASYNC. The kernel resolves write-protect faults itself without queueing an event.
• Uffd::copy_with_mode and CopyMode: exposes the full UFFDIO_COPY mode word. CopyMode::WRITE_PROTECT (behind linux5_7) maps the copied pages write-protected so the retried write raises a write-protect fault. Uffd::copy keeps its signature and delegates to the new method, mirroring the existing register / register_with_mode pair.

Feature levels

The userfaultfd-sys crate already chains its feature levels, but the top-level crate does not, so enabling only linux5_13 leaves cfg(feature = "linux5_7") items such as RegisterMode::WRITE_PROTECT compiled out. The new linux6_4 and linux6_7 features enable every lower level so a single feature selects everything up to that kernel.

I deliberately left existing features linux4_14, linux5_7, and linux5_13 as they are. Chaining them would add the thread_id field to Event::Pagefault and the WriteProtected variant to FaultKind for current users, which could break exhaustive patterns.
If we want to ever fix that, we should do it in a release that bumps major to declare the breaking change.

However, since enabling linux6_4/linux6_7 enables 5.7 and 5.13 together, they expose a non-exhaustive match in test_write_protect, which wasn't there before. So, I've extended it with a FaultKind::Minor arm.

Scope

linux6_7 just brings in write-protect. There are other features that we can add such as UFFDIO_POISON (6.6) or UFFDIO_MOVE (6.8), but I'd do these as separate PR.

As with the existing levels, the new feature bits come from bindgen, so building with linux6_4 or linux6_7 needs kernel headers of at least that version.

Tests

Four new tests, gated on the corresponding feature:

• copy_with_mode with DONTWAKE: the page holds the source bytes before the faulting thread is woken.
• copy_with_mode with WRITE_PROTECT: the retried write raises a write-protect fault with the copied data intact.
• WP_UNPOPULATED: write-protecting a never-touched page makes the first write fault instead of silently populating.
• WP_ASYNC: a write to a protected page succeeds from the same thread and no event is queued, for both an unpopulated and a repopulated page.

I ran those locally on a 7.1 kernel via both /dev/userfaultfd and the syscall fallback, at every feature level.

I added a new linux6_7 step in the CI using the ubuntu-latest runner.

Compatibility

Changes are backwards compatible. Uffd::copy produces the same ioctl mode bits as before. CopyMode, copy_with_mode, and the two feature levels are purely additive, so we don't need a major version bump.