linuxkpi: Migrate to IfAPI

Summary:
Trivial changes for LinuxKPI to use IfAPI. The 'bsdifp' looks unused,
so removed it instead of converting it to a pointer.

Bump __FreeBSD_version for change to struct net_device.

Reviewed by: bz, hselasky
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D39491