Ensure we close netlink sockets

[DL6ER]

What does this implement/fix?

Fix a slow but progressing exhaustion of file descriptors when heavily using https://pi.hole/admin/interfaces by ensuring that each socket is closed after we're done with it.

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Related issue or feature (if applicable): https://discourse.pi-hole.net/t/error-500-internal-server-error-error-cannot-open-script-file-var-www-html-admin-login-lp/73698/

Pull request in docs with documentation (if applicable): N/A

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2. I have commented my proposed changes within the code.
3. I am willing to help maintain this change if there are issues with it later.
4. It is compatible with the EUPL 1.2 license
5. I have squashed any insignificant commits. (git rebase)

Checklist:

• The code change is tested and works locally.
• I based my code and PRs against the repositories developmental branch.
• I signed off all commits. Pi-hole enforces the DCO for all contributions
• I signed all my commits. Pi-hole requires signatures to verify authorship
• I have read the above and my PR is ready for review.

[pralor-bot]

This pull request has been mentioned on Pi-hole Userspace. There might be relevant details there:

https://discourse.pi-hole.net/t/error-500-internal-server-error-error-cannot-open-script-file-var-www-html-admin-login-lp/73698/25