iommufd/selftest: Refactor dirty bitmap tests

Rework the functions that test and set the bitmaps to receive a new
parameter (the pte_page_size) that reflects the expected PTE size in the
page tables. The same scheme is still used i.e. even bits are dirty and
odd page indexes aren't dirty. Here it just refactors to consider the size
of the PTE rather than hardcoded to IOMMU mock base page assumptions.

While at it, refactor dirty bitmap tests to use the idev_id created by the
fixture instead of creating a new one.

This is in preparation for doing tests with IOMMU hugepages where multiple
bits set as part of recording a whole hugepage as dirty and thus the
pte_page_size will vary depending on io hugepages or io base pages.

Link: https://lore.kernel.org/r/20240202133415.23819-6-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>