vfio: fix a missed vfio group put in vfio_pin_pages

When error occurs, need to put vfio group after a successful get.

Fixes: 95fc87b4 ("vfio: Selective dirty page tracking if IOMMU backed device pins pages")
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio.c

@@ -1949,8 +1949,10 @@ int vfio_pin_pages(struct device *dev, unsigned long *user_pfn, int npage,
 	if (!group)
 		return -ENODEV;
 
-	if (group->dev_counter > 1)
-		return -EINVAL;
+	if (group->dev_counter > 1) {
+		ret = -EINVAL;
+		goto err_pin_pages;
+	}
 
 	ret = vfio_group_add_container_user(group);
 	if (ret)