Set correct packet length after defragmenting a packet

[xyzzyz]

When reassembling fragmented packet, we currently don't recompute the total length in the IPv4 header. The comment in the code says that we don't really use the header, but this is not actually true: for raw sockets, this will result in the total_length to be equal to the length in the last received fragment. Users of raw sockets typically parse the IP headers, so it is important to set these correctly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.55%. Comparing base (39cd44e) to head (fe311b0).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1094      +/-   ##
==========================================
+ Coverage   80.45%   80.55%   +0.09%     
==========================================
  Files          81       81              
  Lines       24461    24463       +2     
==========================================
+ Hits        19681    19705      +24     
+ Misses       4780     4758      -22     

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