Revert "bpf: veth driver panics when xdp prog attached before veth_open"

This reverts commit 5e5dc33d.

This patch fixes the panic maked by 2e0de636. Now Paolo
and Toke suggest reverting the patch 2e0de636 and making
it stronger, so do this first.
Signed-off-by: Heng Qi <hengqi@linux.alibaba.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/veth.c

@@ -1125,7 +1125,7 @@ static int veth_enable_xdp(struct net_device *dev)
 	int err, i;
 
 	rq = &priv->rq[0];
-	napi_already_on = rcu_access_pointer(rq->napi);
+	napi_already_on = (dev->flags & IFF_UP) && rcu_access_pointer(rq->napi);
 
 	if (!xdp_rxq_info_is_reg(&priv->rq[0].xdp_rxq)) {
 		err = veth_enable_xdp_range(dev, 0, dev->real_num_rx_queues, napi_already_on);