dnsmasq: use logical interface name for dhcp relay config

The relay section should use the logical interface name and
not the linux network device name directly. This to be
consistent with other sections of the dnsmasq config where
'interface' means the logical interface.

Signed-off-by: Karl Vogel <karl.vogel@gmail.com>

package/network/services/dnsmasq/files/dnsmasq.init

@@ -526,7 +526,8 @@ dhcp_relay_add() {
 	if [ -z "$interface" ]; then
 		xappend "--dhcp-relay=$local_addr,$server_addr"
 	else
-		xappend "--dhcp-relay=$local_addr,$server_addr,$interface"
+		network_get_device ifname "$interface" || return
+		xappend "--dhcp-relay=$local_addr,$server_addr,$ifname"
 	fi
 }