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 14.28571% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 47.93%. Comparing base (dbaaf69) to head (72d7f9d).

Files with missing lines	Patch %	Lines
scapy/layers/dns.py	14.28%	6 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (dbaaf69) and HEAD (72d7f9d). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (dbaaf69)	HEAD (72d7f9d)
	12	2

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #4943       +/-   ##
===========================================
- Coverage   80.28%   47.93%   -32.35%     
===========================================
  Files         375      357       -18     
  Lines       92431    92093      -338     
===========================================
- Hits        74205    44146    -30059     
- Misses      18226    47947    +29721     

Files with missing lines	Coverage Δ
scapy/layers/dns.py	30.58% <14.28%> (-53.03%)	⬇️

... and 327 files with indirect coverage changes

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