iommufd/selftest: Add container_of helpers

iommufd/selftest: Add container_of helpers

Use these inline helpers to shorten those container_of lines.

Note that one of them goes back and forth between iommu_domain and
mock_iommu_domain, which isn't necessary. So drop its container_of.

Link: https://patch.msgid.link/r/518ec64dae2e814eb29fd9f170f58a3aad56c81c.1730836219.git.nicolinc@nvidia.com
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>