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

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 it behaves like the tun device.
This is needed for adding support for the tap device to packetdrill.

Reviewed by: kevans, rscheff
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D42467