fix _packet_subnet calculation logic

[wenxuan70]

Checklist:

• [ x ] If you are new to Scapy: I have checked CONTRIBUTING.md (esp. section submitting-pull-requests)
• [ x ] I squashed commits belonging together
• [ x ] I added unit tests or explained why they are not relevant
• [ x ] I executed the regression tests (using tox)
• [ x ] If the PR is still not finished, please create a Draft Pull Request

fixes #4942

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.30%. Comparing base (dbaaf69) to head (923b03d).
⚠️ Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
scapy/layers/dns.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4943      +/-   ##
==========================================
+ Coverage   80.28%   80.30%   +0.02%     
==========================================
  Files         375      379       +4     
  Lines       92431    93099     +668     
==========================================
+ Hits        74205    74761     +556     
- Misses      18226    18338     +112     

Files with missing lines	Coverage Δ
scapy/layers/dns.py	83.55% <85.71%> (-0.07%)	⬇️

... and 18 files with indirect coverage changes

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[gpotter2]

Hi and thanks for the PR ! It seems that it broke certain test cases, could you take a look?

Thanks

[wenxuan70]

Thanks for letting me know! I'll look into the failing tests and get back to you soon.

[wenxuan70]

The default value of 16 here is to maintain backward compatibility: when scope_plen is None, the original _pack_subnet method computes 16 bits from the default address 192.168.0.0.