net/flow_dissector: add packet hash dissection

Retreive a hash value from the SKB and store it
in the dissector key for future matching.

Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>