iommufd/selftest: Fix iommufd_test_dirty() to handle <u8 bitmaps

iommufd/selftest: Fix iommufd_test_dirty() to handle <u8 bitmaps

The calculation returns 0 if it sets less than the number of bits per
byte. For calculating memory allocation from bits, lets round it up to
one byte.

Link: https://lore.kernel.org/r/20240627110105.62325-3-joao.m.martins@oracle.com
Reported-by: Matt Ochs <mochs@nvidia.com>
Fixes: a9af47e382a4 ("iommufd/selftest: Test IOMMU_HWPT_GET_DIRTY_BITMAP")
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Matt Ochs <mochs@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>