sys/linux/vnet: added descriptions for ipv4 icmp extended headers

[cglosner]

The extended headers are for echo and reply echo messages.

[codecov[bot]]

Codecov Report

Merging #4045 (1ad5923) into master (022df2b) will increase coverage by 0.0%. The diff coverage is n/a.

see 1 file with indirect coverage changes

[a-nogikh]

Please also address this CI error:

Error: Wrong commit subject format: 'AUTHORS/CONTRIBUTORS: Added name'. Please use 'main/affected/package: short change description'. See docs/contributing.md for details.

It's about your first commit's description.