iproute2: set a default broadcast address if none is specified

Reported-by: Spooky Ghost <spookyghost@blueyounder.co.uk>
X-Gentoo-Bug: 392593
X-Gentoo-Bug-URL: http://bugs.gentoo.org/show_bug.cgi?id=392593

net/iproute2.sh

@@ -134,6 +134,9 @@ _add_address()
 		[ -z "$scope" ] && scope="scope host"
 	fi
 
+	# figure out the broadcast address if it is not specified
+	[ -z "$broadcast" ] && broadcast="broadcast +"
+
 	set -- "${address}${netmask}" $peer $broadcast $anycast $label $scope dev "${IFACE}" $valid_lft $preferred_lft $home $nodad
 	veinfo ip addr add "$@"
 	ip addr add "$@"