hv_netvsc: do not use VF device if link is down

If the accelerated networking SRIOV VF device has lost carrier
use the synthetic network device which is available as backup
path. This is a rare case since if VF link goes down, normally
the VMBus device will also loose external connectivity as well.
But if the communication is between two VM's on the same host
the VMBus device will still work.

Reported-by: "Shah, Ashish N" <ashish.n.shah@intel.com>
Fixes: 0c195567a8 ("netvsc: transparent VF management")
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Reviewed-by: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc_drv.c

@@ -532,12 +532,13 @@ static int netvsc_xmit(struct sk_buff *skb, struct net_device *net, bool xdp_tx)
 	u32 hash;
 	struct hv_page_buffer pb[MAX_PAGE_BUFFER_COUNT];
 
-	/* if VF is present and up then redirect packets
-	 * already called with rcu_read_lock_bh
+	/* If VF is present and up then redirect packets to it.
+	 * Skip the VF if it is marked down or has no carrier.
+	 * If netpoll is in uses, then VF can not be used either.
 	 */
 	vf_netdev = rcu_dereference_bh(net_device_ctx->vf_netdev);
 	if (vf_netdev && netif_running(vf_netdev) &&
-	    !netpoll_tx_running(net))
+	    netif_carrier_ok(vf_netdev) && !netpoll_tx_running(net))
 		return netvsc_vf_xmit(net, vf_netdev, skb);
 
 	/* We will atmost need two pages to describe the rndis