bpf: Add IfAPI analogue for bpf_peers_present()

An interface's bpf could feasibly not exist, in which case
bpf_peers_present() would panic from a NULL pointer dereference. Solve
this by adding a new IfAPI that could deal with a NULL bpf, if such
could occur in the network stack.

Reviewed by: zlei
Sponsored by: Juniper Networks, Inc.
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42082