Fix DHCPv6 relay:

Link address write to use longest match prefix address selection.

Change-Id: I8d4c475abc63814450c54113c97d7e9ec812a99d

Author: Juha Heiskanen

source/libDHCPv6/dhcp_service_api.c

@@ -441,7 +441,7 @@ void recv_dhcp_relay_msg(void *cb_res)
         }
         uint8_t gp_address[16];
         //Get blobal address from interface
-        if (arm_net_address_get(sckt_data->interface_id, ADDR_IPV6_GP, gp_address) != 0) {
+        if (addr_interface_select_source(interface_ptr, gp_address, relay_srv->server_address, 0) != 0) {
             // No global prefix available
             tr_error("No GP address");
             goto cleanup;