UDP checksum offload does not work in Azure if following conditions are
met:
- sizeof(IP hdr + UDP hdr + payload) > 1420.
- IP_DF is not set in IP hdr
Use software checksum for UDP datagrams falling into this category.
Add two tunables to disable UDP/IPv4 and UDP/IPv6 checksum offload, in
case something unexpected happened.
MFC after: 1 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D12429
b69ef721ad