[ipv6] Include network device when transcribing multicast addresses

Destination multicast addresses require a sin6_scope_id, which should
therefore be transcribed to a network device name by ipv6_sock_ntoa().

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/net/ipv6.c

 	const char *netdev_name;
 
 	/* Identify network device, if applicable */
-	if ( IN6_IS_ADDR_LINKLOCAL ( in ) ) {
+	if ( IN6_IS_ADDR_LINKLOCAL ( in ) || IN6_IS_ADDR_MULTICAST ( in ) ) {
 		netdev = find_netdev_by_index ( sin6->sin6_scope_id );
 		netdev_name = ( netdev ? netdev->name : "UNKNOWN" );
 	} else {