nfp: flower-ct: add pre and post ct checks

Add checks to see if a flow is a conntrack flow we can potentially
handle. Just stub out the handling the different conntrack flows.

Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>