[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/ipv4.c

@@ -271,7 +271,7 @@ static int ipv4_ll_addr ( struct in_addr dest, struct in_addr src,
 
 	if ( dest.s_addr == INADDR_BROADCAST ) {
 		/* Broadcast address */
-		memcpy ( ll_dest, ll_protocol->ll_broadcast,
+		memcpy ( ll_dest, netdev->ll_broadcast,
 			 ll_protocol->ll_addr_len );
 		return 0;
 	} else if ( IN_MULTICAST ( ntohl ( dest.s_addr ) ) ) {