let tun use bpf_mtap for handling input packets.

let tun use bpf_mtap for handling input packets.

tun (not tap) input packets are written from userland in the same
format that it's bpf dlt is expecting, so we can push the packet
straight into bpf with bpf_mtap. this is more correct that using
bpf_mtap_ether for tun.