dns: fix EDNS0ClientSubnet address truncation for non-octet-aligned prefix lengths

[T3pp31]

Summary

Fixes #4942.

EDNS0ClientSubnet._pack_subnet had two related bugs when source_plen is set explicitly:

1. Wrong byte count — it stripped trailing zero bytes rather than using ceil(source_plen / 8), so source_plen=23 on 101.132.0.0 produced a 2-byte address instead of 3.
2. Host bits not zeroed — even after the byte count is correct, the final partial byte must have its host bits cleared per RFC 7871. For example source_plen=23 on 101.132.255.0 must produce 0xfe as the third byte, not 0xff.

Per RFC 7871 §6:

ADDRESS … MUST be truncated to the number of bits indicated by the SOURCE PREFIX-LENGTH field, padded with 0 bits to pad to the end of the last octet needed.

Changes

• _pack_subnet now accepts an optional plen parameter. When provided it truncates to exactly ceil(plen / 8) bytes and masks off host bits in the last partial byte when plen % 8 != 0.
• i2m and i2len pass pkt.source_plen so explicit prefix lengths are respected during packet building.
• When plen is None (auto-detect mode) the original trailing-zero-stripping behaviour is preserved.

Reproduction

from scapy.layers.dns import EDNS0ClientSubnet

# Bug 1: wrong byte count
b = EDNS0ClientSubnet(source_plen=23, address='101.132.0.0')
# Before: 00080006000117006584    (2-byte address, optlen=6)
# After:  000800070001170065840 0  (3-byte address, optlen=7)

# Bug 2: host bits not zeroed
b = EDNS0ClientSubnet(source_plen=23, address='101.132.255.0')
# Before: 000800070001170065 84ff  (0xff — host bit set)
# After:  000800070001170065 84fe  (0xfe — host bit cleared)

Tests

Five regression tests added to test/scapy/layers/dns_edns0.uts:

• source_plen=23 with trailing-zero last byte (original report)
• source_plen=24 octet-aligned sanity check
• source_plen=0 produces empty address field
• source_plen=23 with host bits set in last byte (IPv4)
• source_plen=33 with host bits set in last byte (IPv6)

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.30%. Comparing base (9ae49f8) to head (a7076c7).

Files with missing lines	Patch %	Lines
scapy/layers/dns.py	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4960      +/-   ##
==========================================
- Coverage   80.30%   80.30%   -0.01%     
==========================================
  Files         379      379              
  Lines       93164    93173       +9     
==========================================
- Hits        74820    74818       -2     
- Misses      18344    18355      +11     

Files with missing lines	Coverage Δ
scapy/layers/dns.py	83.79% <92.85%> (+0.17%)	⬆️

... and 10 files with indirect coverage changes

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