[IPIP]: Make the fallback tunnel device per-net.

Create on in ipip_init_net(), use it all over the code (the
proper place to get the net from already exists) and destroy
in ipip_net_exit().

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