bugfix CU/DU: use correct IP address for GTP-U

Not only the teid is received from the DU but also the IP address. We have to use it too.

Also, don't connect the UDP socket. The parameter remote_s_address in the configuration file must not be used, at least not for gtp-u because the IP address is received from the DU (plus for later, for multiple DUs, it won't work at all).