if_tuntap: trigger the bpf hook on transmitting for the tap interface

[tuexen]

Summary

The tun interface triggers the bpf hook when a packet is transmitted, the tap interface triggers it when the packet is read from the character device. This is inconsistent. So fix the tap device such that is behaves like the tun device. This is needed for adding support for the tap device to packetdrill.

[rgrimes]

This looks appropriate to me.