[NETFILTER]: nf_nat_proto_gre: do not modify/corrupt GREv0 packets through NAT

While porting some changes of the 2.6.21-rc7 pptp/proto_gre conntrack
and nat modules to a 2.4.32 kernel I noticed that the gre_key function
returns a wrong pointer to the GRE key of a version 0 packet thus
corrupting the packet payload.

The intended behaviour for GREv0 packets is to act like
nf_conntrack_proto_generic/nf_nat_proto_unknown so I have ripped the
offending functions (not used anymore) and modified the
nf_nat_proto_gre modules to not touch version 0 (non PPTP) packets.

Signed-off-by: Jorge Boncompte <jorge@dti2.net>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>