Move the definition of struct bpf_if to bpf.c.

A couple of fields are still exposed via struct bpf_if_ext so that
bpf_peers_present() can be inlined into its callers. However, this change
eliminates some type duplication in the resulting CTF container, since
otherwise ctfmerge(1) propagates the duplication through all types that
contain a struct bpf_if.

Differential Revision: https://reviews.freebsd.org/D2319
Reviewed by: melifaro, rpaulo