[GRE]: Add net/gre_net argument to some functions.

The fallback device and hashes are to become per-net, but many
code doesn't have anything to get the struct net pointer from.

So pass the proper net there with an extra argument.

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