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:	#322
Obtained from:	FreeBSD

sys/dev/netmap/netmap_generic.c

@@ -75,6 +75,7 @@ __FBSDID("$FreeBSD: head/sys/dev/netmap/netmap_generic.c 274353 2014-11-10 20:19
 #include <sys/socket.h> /* sockaddrs */
 #include <sys/selinfo.h>
 #include <net/if.h>
+#include <net/if_types.h>
 #include <net/if_var.h>
 #include <machine/bus.h>        /* bus_dmamap_* in netmap_kern.h */
 
@@ -1198,6 +1199,13 @@ generic_netmap_attach(struct ifnet *ifp)
 	int retval;
 	u_int num_tx_desc, num_rx_desc;
 
+#ifdef __FreeBSD__
+	if (ifp->if_type == IFT_LOOP) {
+		D("if_loop is not supported by %s", __func__);
+		return EINVAL;
+	}
+#endif
+
 	num_tx_desc = num_rx_desc = netmap_generic_ringsize; /* starting point */
 
 	nm_os_generic_find_num_desc(ifp, &num_tx_desc, &num_rx_desc); /* ignore errors */