net: pkt: IP tunneling needs to store remote address

Use the "remote" sockaddr field for storing the remote
tunnel end point address. This is used also by offloading but
tunneling and offloading are not used at the same time for the
same interface so sharing the variable will not conflict.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>