netvsc: fix race during initialization

When device is being setup on boot, there is a small race where
network device callback is registered, but the netvsc_device pointer
is not set yet.  This can cause a NULL ptr dereference if packet
arrives during this window.

Fixes: 46b4f7f5d1f7 ("netvsc: eliminate per-device outstanding send counter")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/hyperv/netvsc.c

@@ -1231,8 +1231,11 @@ void netvsc_channel_cb(void *context)
 		return;
 
 	net_device = net_device_to_netvsc_device(ndev);
-	if (unlikely(net_device->destroy) &&
-	    netvsc_channel_idle(net_device, q_idx))
+	if (unlikely(!net_device))
+		return;
+
+	if (unlikely(net_device->destroy &&
+		     netvsc_channel_idle(net_device, q_idx)))
 		return;
 
 	/* commit_rd_index() -> hv_signal_on_read() needs this. */