netback: don't store invalid vif pointer

When xenvif_alloc() fails, it returns a non-NULL error indicator. To
avoid eventual races, we shouldn't store that into struct backend_info
as readers of it only check for NULL.
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/xen-netback/xenbus.c

@@ -404,6 +404,7 @@ static int backend_create_xenvif(struct backend_info *be)
 	int err;
 	long handle;
 	struct xenbus_device *dev = be->dev;
+	struct xenvif *vif;
 
 	if (be->vif != NULL)
 		return 0;
@@ -414,13 +415,13 @@ static int backend_create_xenvif(struct backend_info *be)
 		return (err < 0) ? err : -EINVAL;
 	}
 
-	be->vif = xenvif_alloc(&dev->dev, dev->otherend_id, handle);
-	if (IS_ERR(be->vif)) {
-		err = PTR_ERR(be->vif);
-		be->vif = NULL;
+	vif = xenvif_alloc(&dev->dev, dev->otherend_id, handle);
+	if (IS_ERR(vif)) {
+		err = PTR_ERR(vif);
 		xenbus_dev_fatal(dev, err, "creating interface");
 		return err;
 	}
+	be->vif = vif;
 
 	kobject_uevent(&dev->dev.kobj, KOBJ_ONLINE);
 	return 0;