MFC r332812:
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().
MFC r332949 (by markj):
Use dead_bpf_if instead of bp_null. This fixes a -Wunused error when DEV_BPF and NETGRAPH_BPF are not defined. Also remove a stray semicolon added in r332812.