cls_flow: use skb_flow_dissect()

Instead of using a custom flow dissector, use skb_flow_dissect() and
benefit from tunnelling support.

This lack of tunnelling support was mentioned by Dan Siemon.

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>