[netdevice] Make ll_broadcast per-netdevice rather than per-ll_protocol

IPoIB has a link-layer broadcast address that varies according to the
partition key.  We currently go through several contortions to pretend
that the link-layer address is a fixed constant; by making the
broadcast address a property of the network device rather than the
link-layer protocol it will be possible to simplify IPoIB's broadcast
handling.

src/net/arp.c

@@ -156,7 +156,7 @@ int arp_resolve ( struct net_device *netdev, struct net_protocol *net_protocol,
 
 	/* Transmit ARP request */
 	if ( ( rc = net_tx ( iobuf, netdev, &arp_protocol, 
-			     ll_protocol->ll_broadcast ) ) != 0 )
+			     netdev->ll_broadcast ) ) != 0 )
 		return rc;
 
 	return -ENOENT;