Two flows can be merged if the second flow (after recirculation) matches
on bits that are either matched on or explicitly set by the first flow.
This means that if a packet hits flow 1 and recirculates then it is
guaranteed to hit flow 2.

Add a 'can_merge' function that determines if 2 sub_flows in a merge hint
can be validly merged to a single flow.
Signed-off-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>