iommufd/viommu: Roll back to use iommufd_object_alloc() for vdevice

iommufd/viommu: Roll back to use iommufd_object_alloc() for vdevice

To solve the vdevice lifecycle issue, future patches make the vdevice
allocation protected by lock. That will make
_iommufd_object_alloc_ucmd() not applicable for vdevice. Roll back to
use _iommufd_object_alloc() for preparation.

Link: https://patch.msgid.link/r/20250716070349.1807226-2-yilun.xu@linux.intel.com
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Xu Yilun <yilun.xu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>