Lift IFA_ -constants from linux/gnu to linux

[jtt]

Description

Lift IFA_ -constants from linux/gnu to linux/mod.rs

Sources

Constants are defined in linux/if_addr.h

Checklist

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

[rustbot]

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[tgross35]

Since these are uapi, could you just lift the existing gnu definitions to Linux/mod.rs?

[jtt]

Lifted to constants to linux/mod.rs and updated the wording of this PR to reflect that. I would also like to nominate this change to be included in stable branch. @rustbot label stable-nominated

[tgross35]

LGTM, thanks!