Do not allow the use of the loopback interface in netmap.

The generic support in netmap send the packets using if_transmit() and the
loopback do not support packets coming from if_transmit()/if_start().

This avoids the use of the loopback interface and the subsequent crash that
happens when the application send packets to the loopback interface.

Details in: https://github.com/luigirizzo/netmap/issues/322
Reported by: Vincenzo Maffione <v.maffione@gmail.com>
Sponsored by: Rubicon Communications, LLC (Netgate)