iommufd/selftest: Fix page leaks in mock_viommu_{init,destroy}

iommufd/selftest: Fix page leaks in mock_viommu_{init,destroy}

mock_viommu_init() allocates two pages using __get_free_pages(..., 1),
but its error path and mock_viommu_destroy() only release the first page
using free_page(), leaking the second page. Use free_pages() with the
matching order instead to avoid any page leaks.

Fixes: 80478a2b450e ("iommufd/selftest: Add coverage for the new mmap interface")
Link: https://patch.msgid.link/r/20260312164040.457293-3-thorsten.blum@linux.dev
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>