MINOR: sock_inet: implement sock_inet_get_dst()

MINOR: sock_inet: implement sock_inet_get_dst()

This one is common to the TCPv4 and UDPv4 code, it retrieves the
destination address of a socket, taking care of the possiblity that for
an incoming connection the traffic was possibly redirected. The TCP and
UDP definitions were updated to rely on it and remove duplicated code.