iommufd/iova_bitmap: Cache mapped length in iova_bitmap_map struct

iommufd/iova_bitmap: Cache mapped length in iova_bitmap_map struct

The amount of IOVA mapped will be used more often in iova_bitmap_set() in
preparation to dynamically iterate the bitmap. Cache said length to avoid
having to calculate it all the time.

Link: https://lore.kernel.org/r/20240627110105.62325-8-joao.m.martins@oracle.com
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>