validate Linkname for insecure paths in Reader.Next

[metsw24-max]

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

This creates an inconsistency where a tar entry can:

• pass validation via Header.Name
• include a crafted Linkname
• bypass the intended path safety checks

Since Header.Linkname is used when processing symlinks and hardlinks it is equally security-sensitive and can influence filesystem
resolution during extraction in common usage patterns.

Additionally:

• Normalizes Linkname using filepath.Clean before validation
• Adds a regression test to ensure insecure Linkname values trigger ErrInsecurePath without affecting subsequent entries

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