bpf: Add missing NOP stubs

This fixes kernel build with nodevice bpf [1].

[1] https://lists.freebsd.org/archives/freebsd-current/2023-February/003178.html

Reported by: Gary Jennejohn <garyj@gmx.de>
Reviewed by: jhibbits
Fixes: 950cc1f44fbd bpf: Add "_if" tap APIs
Differential Revision: https://reviews.freebsd.org/D38432