Add example to ng_bpf(4) showing how ng_bpf(4) can be used to prevent spoofing.

Differential Revision: https://reviews.freebsd.org/D26488
MFC after: 1 week
Reviewed by: pi, bcr (manpages)
Submitted by: lutz_donnerhacke.de (Lutz Donnerhacke)
Sponsored by: Mellanox Technologies // NVIDIA Networking