iommufd/selftest: Add mock_domain_cache_invalidate_user support

Add mock_domain_cache_invalidate_user() data structure to support user
space selftest program to cover user cache invalidation pathway.

Link: https://lore.kernel.org/r/20240111041015.47920-5-yi.l.liu@intel.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Co-developed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>