ovs: clear skb->tstamp in forwarding path

fq qdisc requires tstamp to be cleared in the forwarding path. Now ovs
doesn't clear skb->tstamp. We encountered a problem with linux
version 5.4.56 and ovs version 2.14.1, and packets failed to
dequeue from qdisc when fq qdisc was attached to ovs port.

Fixes: fb420d5 ("tcp/fq: move back to CLOCK_MONOTONIC")
Signed-off-by: kaixi.fan <fankaixi.li@bytedance.com>
Signed-off-by: xiexiaohui <xiexiaohui.xxh@bytedance.com>
Reviewed-by: Cong Wang <cong.wang@bytedance.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/openvswitch/vport.c

@@ -507,6 +507,7 @@ void ovs_vport_send(struct vport *vport, struct sk_buff *skb, u8 mac_proto)
 	}
 
 	skb->dev = vport->dev;
+	skb->tstamp = 0;
 	vport->ops->send(skb);
 	return;