[IPIP]: Add net/ipip_net argument to some functions.

The hashes of tunnels will be per-net too, so prepare all the
functions that uses them for this change by adding an argument.

Use init_net temporarily in places, where the net does not exist
explicitly yet.

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