iommufd/iova_bitmap: Consider page offset for the pages to be pinned

For small bitmaps that aren't PAGE_SIZE aligned *and* that are less than
512 pages in bitmap length, use an extra page to be able to cover the
entire range e.g. [1M..3G] which would be iterated more efficiently in a
single iteration, rather than two.

Fixes: b058ea3a

 ("vfio/iova_bitmap: refactor iova_bitmap_set() to better handle page boundaries")
Link: https://lore.kernel.org/r/20240202133415.23819-10-joao.m.martins@oracle.com
Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
Tested-by: Avihai Horon <avihaih@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>