inet: choose the payload class from reassembled fragments

[KernelClint]

_defrag_ip_pkt() puts a fragmented IPv4 datagram back together and then has to decide what the
reassembled bytes are. At
scapy/layers/inet.py:1467-1505
it reuses the class Scapy guessed for the first fragment's payload:

pay_class = p[IP].payload.__class__

The first fragment is often too short to classify correctly — with a small enough first fragment
Scapy has not seen a complete TCP header, so the guess is whatever it managed from those bytes. The
complete datagram is then forced into that class regardless of what the assembled bytes actually
are. Through TCPSession, an HTTP response that arrived in valid ordered fragments comes back
without its application layer, while the byte-identical unfragmented response comes back correctly.

The change asks the reassembled IP layer to classify the complete bytes, the same way ordinary
dissection does:

-            pay_class = p[IP].payload.__class__
+            pay_class = p[IP].guess_payload_class(data)
...
-            p /= pay_class(data)
+            p[IP].add_payload(pay_class(data))

Every valid fragment layout is still reassembled; what changes is that the result is classified from
what was actually received.

The added regression reassembles the same HTTP response at two fragment sizes, including one whose
first fragment is too small to classify, and asserts the response body is delivered in both. Without
the source change it fails.

Performance was measured on one computer, before and after the fix: reassembling a valid ordered
datagram took 232.0 µs before and 176.3 µs after — 24% faster. Repeat runs moved by less than
that, so it is a real difference. Dispatching normally avoids the work the old path spent forcing
bytes into a class that could not parse them.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.64%. Comparing base (b3bbcc8) to head (4bc5e3f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5143   +/-   ##
=======================================
  Coverage   80.63%   80.64%           
=======================================
  Files         390      390           
  Lines       96936    96936           
=======================================
+ Hits        78168    78174    +6     
+ Misses      18768    18762    -6     

Files with missing lines	Coverage Δ
scapy/layers/inet.py	71.94% <100.00%> (ø)

... and 5 files with indirect coverage changes

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