netfilter: nf_flow_table: detach routing information from flow description

This patch adds the infrastructure to support for flow entry types.
The initial type is NF_FLOW_OFFLOAD_ROUTE that stores the routing
information into the flow entry to define a fastpath for the classic
forwarding path.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>