log and drop ipv6 packets requiring fragmentation

[Easyoakland]

Currently packets which exceed the MTU are still transmitted for ipv6 without regard for their size.

This pr changes the behavior for ipv6 to match the behavior for ipv4: drop packet and net_debug instead of trying to transmit. This prevents panics in Device implementations that assume the given packet doesn't exceed an MTU in size.

[codecov]

Codecov Report

Attention: Patch coverage is 78.57143% with 3 lines in your changes missing coverage. Please review.

Project coverage is 80.84%. Comparing base (49cc60a) to head (bef9b05).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/iface/interface/mod.rs	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1038      +/-   ##
==========================================
- Coverage   80.84%   80.84%   -0.01%     
==========================================
  Files          81       81              
  Lines       28481    28485       +4     
==========================================
+ Hits        23026    23029       +3     
- Misses       5455     5456       +1     

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[whitequark]

Looks good to me other than the comment!

[whitequark]

Thanks! If you squash the commits I'll hit merge.

[Easyoakland]

Done.

[whitequark]

Thanks for the PR and the prompt response!