vfio: Prevent open_count decrement to negative

When vfio_df_close() is called with open_count=0, it triggers a warning in
vfio_assert_device_open() but still decrements open_count to -1. This allows
a subsequent open to incorrectly pass the open_count == 0 check, leading to
unintended behavior, such as setting df->access_granted = true.

For example, running an IOMMUFD compat no-IOMMU device with VFIO tests
(https://github.com/awilliam/tests/blob/master/vfio-noiommu-pci-device-open.c)
results in a warning and a failed VFIO_GROUP_GET_DEVICE_FD ioctl on the first
run, but the second run succeeds incorrectly.

Add checks to avoid decrementing open_count below zero.

Fixes: 05f37e1 ("vfio: Pass struct vfio_device_file * to vfio_device_open/close()")
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jacob Pan <jacob.pan@linux.microsoft.com>
Link: https://lore.kernel.org/r/20250618234618.1910456-2-jacob.pan@linux.microsoft.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

Author: jacobpan

drivers/vfio/vfio_main.c

@@ -583,7 +583,8 @@ void vfio_df_close(struct vfio_device_file *df)
 
 	lockdep_assert_held(&device->dev_set->lock);
 
-	vfio_assert_device_open(device);
+	if (!vfio_assert_device_open(device))
+		return;
 	if (device->open_count == 1)
 		vfio_df_device_last_close(df);
 	device->open_count--;