Remove the assumption that skb_get_rxhash() makes IP header and ports
linear, and use skb_header_pointer() instead in choke_match_flow()

This permits __skb_get_rxhash() to use skb_header_pointer() eventually.
Signed-off-by: NEric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>