gtp: switch from struct socket to struct sock for the GTP sockets

After enabling the UDP encapsulation, only the sk member is used.

Holding the socket would prevent user space from closing the socket,
but holding a reference to the sk member does not have the same
effect.

This change will make it simpler to later detach the sockets from
the netdevice.

Signed-off-by: Andreas Schultz <aschultz@tpip.net>
Signed-off-by: David S. Miller <davem@davemloft.net>