archive/tar: validate Linkname for insecure paths in Reader.Next

[metsw24-max]

The archive/tar Reader validates Header.Name for insecure paths when
GODEBUG=tarinsecurepath=0, but does not apply the same validation to
Header.Linkname.

This means an entry can pass the Header.Name check while carrying a
crafted Linkname that bypasses the intended path safety checks. Since
Header.Linkname is used when creating symlinks and hard links, it can
influence filesystem resolution during extraction in common usage
patterns, so it is equally security-sensitive.

Validate Header.Linkname with filepath.IsLocal in Reader.Next, matching
the existing Header.Name check, and return ErrInsecurePath when it is
non-local. Empty link names are skipped. Also add a regression test to
ensure an insecure Linkname triggers ErrInsecurePath without affecting
subsequent entries.

Updates #55356

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[gopherbot]

Message from Gopher Robot:

Patch Set 1:

(1 comment)

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[metsw24-max]

Updated the PR title and description per the robot's findings: added the archive/tar package prefix, wrapped the body at ~76 columns and referenced #55356. Also pushed a follow-up commit that reuses the existing filepath.IsLocal check for Linkname (the separate Clean step was redundant) and fixes formatting, so the change is now a fair bit smaller.

[gopherbot]

This PR (HEAD: 5c96d9b) has been imported to Gerrit for code review.

Please visit Gerrit at https://go-review.googlesource.com/c/go/+/772900.

Important tips:

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[gopherbot]

This PR is being closed because golang.org/cl/772900 has been abandoned.

Sorry, but this isn't a change we should make.