feat: add ioctl flags in linux/if_tun.h

[wlynxg]

Description

Add ioctl flags from linux/if_tun.h

Sources

https://github.com/torvalds/linux/blob/d3d1556696c1a993eec54ac585fe5bf677e07474/include/uapi/linux/if_tun.h#L31

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @tgross35 (or someone else) some time within the next two weeks.

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