x-pack/auditbeat/module/system/process: don't try to hash files in other namespaces

[efd6]

What does this PR do?

This change prevents fruitless attempts to hash files in mnt namespaces that differ from the running auditbeat process.

Why is it important?

See linked related issues.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
  - [ ] I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

Author's Checklist

N/A

How to test this PR locally

mage test

Related issues

• Relates auditbeat doesn't recognize that a path in an audit record might be in an inaccessible namespace. #25777
• Relates Auditbeat running on host can't hash executable inside container - different namespaces #29678

Use cases

See linked issues.

Screenshots

N/A

Logs

N/A

[elasticmachine]

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• Start Time: 2022-01-11T06:40:55.526+0000

• Duration: 69 min 11 sec

• Commit: b819121

Test stats 🧪

Test	Results
Failed	0
Passed	342
Skipped	145
Total	487

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[elasticmachine]

Pinging @elastic/security-external-integrations (Team:Security-External Integrations)

[adriansr]

LGTM, thanks for fixing