Add dead_bpf_if structure, that should be used as fake bpf_if
during ifnet detach.

Since destroying interface is not atomic operation and due to the
lack of synhronization during destroy, it is possible, that in the
time between bpfdetach() and if_free() some queued on destroying
interface mbuf will be used by ether_input_internal() and bpf_peers_present()
can dereference NULL bpf_if pointer. To protect from this, assign
pointer to empty bpf_if_ext structure instead of NULL pointer after
bpfdetach().