Fix the error of specific length IP packets not being fragmented

[Jc0x7D3]

There is an obvious mistake at src/iface/interface/mod.rs:1204: total_ip_len represents the length of the IP packet, while self.caps.max_transmission_unit is the max length of link-layer packets. The correct comparison should be with self.caps.ip_mtu().

When using Medium Ethernet, since self.caps.max_transmission_unit is 14 octets larger than self.caps.ip_mtu(), IP packets with length of 1501~1514 octets should be fragmented, but mistakenly entered the non-fragment branch.

Additionally, I added a test case sending IPv4 packets of different lengths, which can trigger this error.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.69%. Comparing base (336e7e5) to head (aba9add).
Report is 30 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1008      +/-   ##
==========================================
- Coverage   80.73%   80.69%   -0.05%     
==========================================
  Files          81       81              
  Lines       28335    28256      -79     
==========================================
- Hits        22877    22801      -76     
+ Misses       5458     5455       -3     

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[Dirbaio]

thanks!