iommufd/selftest: Refactor mock_domain_read_and_clear_dirty()

Move the clearing of the dirty bit of the mock domain into
mock_domain_test_and_clear_dirty() helper, simplifying the caller
function.

Additionally, rework the mock_domain_read_and_clear_dirty() loop to
iterate over a potentially variable IO page size. No functional change
intended with the loop refactor.

This is in preparation for dirty tracking support for IOMMU hugepage mock
domains.

Link: https://lore.kernel.org/r/20240202133415.23819-7-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>