nfp: add metadata to each flow offload

Adds metadata describing the mask id of each flow and keeps track of
flows installed in hardware. Previously a flow could not be removed
from hardware as there was no way of knowing if that a specific flow
was installed. This is solved by storing the offloaded flows in a
hash table.

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>