net_sched: cls_flow: add key rxhash

We can use rxhash to classify the traffic into flows. As rxhash maybe
supplied by NIC or RPS, it is cheaper.

Signed-off-by: Changli Gao <xiaosuo@gmail.com>
Acked-by: Jamal Hadi Salim <hadi@cyberus.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>