vfio: fix crash on rmmod

devtmpfs_delete_node() calls devnode() callback with mode==NULL but
vfio still tries to write there.

The patch fixes this.
Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio.c

@@ -1360,7 +1360,7 @@ static const struct file_operations vfio_device_fops = {
  */
 static char *vfio_devnode(struct device *dev, umode_t *mode)
 {
-	if (MINOR(dev->devt) == 0)
+	if (mode && (MINOR(dev->devt) == 0))
 		*mode = S_IRUGO | S_IWUGO;
 
 	return kasprintf(GFP_KERNEL, "vfio/%s", dev_name(dev));