[IPIP]: Make tunnels hashes per net.

Either net or ipip_net already exists in all the required
places, so just use one.

Besides, tune net_init and net_exit calls to respectively
initialize the hashes and destroy devices.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>