[ipv4] Record ARP resolution errors

At the time of attempting ARP resolution, we already know the
transmitting network device.  We can therefore record ARP errors using
netdev_tx_err() so that they show up in the output of "ifstat".

Inspired-by: Dominik Russenberger <dominik.russenberger@terreactive.ch>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv4.c

 				   ll_dest ) ) != 0 ) {
 		DBG ( "IPv4 has no link-layer address for %s: %s\n",
 		      inet_ntoa ( next_hop ), strerror ( rc ) );
+		/* Record error for diagnosis */
+		netdev_tx_err ( netdev, iob_disown ( iobuf ), rc );
 		goto err;
 	}