iommufd/selftest: Add mock IO hugepages tests

Leverage previously added MOCK_FLAGS_DEVICE_HUGE_IOVA flag to create an
IOMMU domain with more than MOCK_IO_PAGE_SIZE supported.

Plumb the hugetlb backing memory for buffer allocation and change the
expected page size to MOCK_HUGE_PAGE_SIZE (1M) when hugepage variant test
cases are used. These so far are limited to 128M and 256M IOVA range tests
cases which is when 1M hugepages can be used.

Link: https://lore.kernel.org/r/20240202133415.23819-9-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>