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

This is the part#2 of the patch #2 - get the proper net for
these functions. This change in a separate patch in order not
to get lost in a large previous patch.

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