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