nfp: flower: ignore duplicate merge hints from FW

A merge hint message needs some time to process before the merged
flow actually reaches the firmware, during which we may get duplicate
merge hints if there're more than one packet that hit the pre-merged
flow. And processing duplicate merge hints will cost extra host_ctx's
which are a limited resource.

Avoid the duplicate merge by using hash table to store the sub_flows
to be merged.

Fixes: 8af56f40

 ("nfp: flower: offload merge flows")
Signed-off-by: Yinjun Zhang <yinjun.zhang@corigine.com>
Signed-off-by: Louis Peens <louis.peens@corigine.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>