freebsd: import if_ptnet change from upstream
freebsd: import if_ptnet change from upstream

Align if_ptnet to the changes introduced by r347233

This removes non-functional SCTP checksum offload support.
More information in the log message of r347233.

luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap,luigirizzo/netmap