nfp: flower-ct: fill in ct merge check function

Replace merge check stub code with the actual implementation. This
checks that the match parts of two tc flows does not conflict.
Only overlapping keys needs to be checked, and only the narrowest
masked parts needs to be checked, so each key is masked with the
AND'd result of both masks before comparing.

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>