[IPIP]: Use proper net in hash-lookup functions.

This is the part#2 of the previous patch - get the proper
net for these functions.

I make it in a separate patch, so that this change does not
get lost in a large previous patch.

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