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: fb420d5d

 ("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>