[NETFILTER]: Use conntrack information to determine if packet was NATed

Preparation for IPsec support for NAT:
Use conntrack information instead of saving the saving and comparing the
addresses to determine if a packet was NATed and needs to be rerouted to
make it easier to extend the key.

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