iommufd/selftest: Hugepage mock domain support

Add support to mock iommu hugepages of 1M (for a 2K mock io page size). To
avoid breaking test suite defaults, the way this is done is by explicitly
creating a iommu mock device which has hugepage support (i.e. through
MOCK_FLAGS_DEVICE_HUGE_IOVA).

The same scheme is maintained of mock base page index tracking in the
XArray, except that an extra bit is added to mark it as a hugepage. One
subpage containing the dirty bit, means that the whole hugepage is dirty
(similar to AMD IOMMU non-standard page sizes). For clearing, same thing
applies, and it must clear all dirty subpages.

This is in preparation for dirty tracking to mark mock hugepages as
dirty to exercise all the iova-bitmap fixes.

Link: https://lore.kernel.org/r/20240202133415.23819-8-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>