iommufd/selftest: Remove domain_alloc_paging()

Since this implements domain_alloc_paging_flags() it only needs one
op. Fold mock_domain_alloc_paging() into mock_domain_alloc_paging_flags().

Link: https://patch.msgid.link/r/0-v1-8a3e7e21ff6a+1745d-iommufd_paging_flags_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>