vfio: Fix unbalanced vfio_df_close call in no-iommu mode

For devices with no-iommu enabled in IOMMUFD VFIO compat mode, the group open
path skips vfio_df_open(), leaving open_count at 0. This causes a warning in
vfio_assert_device_open(device) when vfio_df_close() is called during group
close.

The correct behavior is to skip only the IOMMUFD bind in the device open path
for no-iommu devices. Commit 6086efe omitted vfio_df_open(), which was
too broad. This patch restores the previous behavior, ensuring
the vfio_df_open is called in the group open path.

Fixes: 6086efe ("vfio-iommufd: Move noiommu compat validation out of vfio_iommufd_bind()")
Suggested-by: Alex Williamson <alex.williamson@redhat.com>
Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20250618234618.1910456-1-jacob.pan@linux.microsoft.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: jacobpan

drivers/vfio/group.c

@@ -192,11 +192,10 @@ static int vfio_df_group_open(struct vfio_device_file *df)
 		 * implies they expected translation to exist
 		 */
 		if (!capable(CAP_SYS_RAWIO) ||
-		    vfio_iommufd_device_has_compat_ioas(device, df->iommufd))
+		    vfio_iommufd_device_has_compat_ioas(device, df->iommufd)) {
 			ret = -EPERM;
-		else
-			ret = 0;
-		goto out_put_kvm;
+			goto out_put_kvm;
+		}
 	}
 
 	ret = vfio_df_open(df);

drivers/vfio/iommufd.c

@@ -25,6 +25,10 @@ int vfio_df_iommufd_bind(struct vfio_device_file *df)
 
 	lockdep_assert_held(&vdev->dev_set->lock);
 
+	/* Returns 0 to permit device opening under noiommu mode */
+	if (vfio_device_is_noiommu(vdev))
+		return 0;
+
 	return vdev->ops->bind_iommufd(vdev, ictx, &df->devid);
 }