xen-netback: do not report success if backend_create_xenvif() fails

If xenvif_alloc() or xenbus_scanf() fail in backend_create_xenvif(),
xenbus is left in offline mode but netback_probe() reports success.

The patch implements propagation of error code for backend_create_xenvif().

Found by Linux Driver Verification project (linuxtesting.org).
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Acked-by: Wei Liu <wei.liu2@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/xenbus.c

@@ -39,7 +39,7 @@ struct backend_info {
 static int connect_rings(struct backend_info *be, struct xenvif_queue *queue);
 static void connect(struct backend_info *be);
 static int read_xenbus_vif_flags(struct backend_info *be);
-static void backend_create_xenvif(struct backend_info *be);
+static int backend_create_xenvif(struct backend_info *be);
 static void unregister_hotplug_status_watch(struct backend_info *be);
 static void set_backend_state(struct backend_info *be,
 			      enum xenbus_state state);
@@ -352,7 +352,9 @@ static int netback_probe(struct xenbus_device *dev,
 	be->state = XenbusStateInitWait;
 
 	/* This kicks hotplug scripts, so do it immediately. */
-	backend_create_xenvif(be);
+	err = backend_create_xenvif(be);
+	if (err)
+		goto fail;
 
 	return 0;
 
@@ -397,19 +399,19 @@ static int netback_uevent(struct xenbus_device *xdev,
 }
 
 
-static void backend_create_xenvif(struct backend_info *be)
+static int backend_create_xenvif(struct backend_info *be)
 {
 	int err;
 	long handle;
 	struct xenbus_device *dev = be->dev;
 
 	if (be->vif != NULL)
-		return;
+		return 0;
 
 	err = xenbus_scanf(XBT_NIL, dev->nodename, "handle", "%li", &handle);
 	if (err != 1) {
 		xenbus_dev_fatal(dev, err, "reading handle");
-		return;
+		return (err < 0) ? err : -EINVAL;
 	}
 
 	be->vif = xenvif_alloc(&dev->dev, dev->otherend_id, handle);
@@ -417,10 +419,11 @@ static void backend_create_xenvif(struct backend_info *be)
 		err = PTR_ERR(be->vif);
 		be->vif = NULL;
 		xenbus_dev_fatal(dev, err, "creating interface");
-		return;
+		return err;
 	}
 
 	kobject_uevent(&dev->dev.kobj, KOBJ_ONLINE);
+	return 0;
 }
 
 static void backend_disconnect(struct backend_info *be)