iommupt: Add iova_to_phys op

iommupt: Add iova_to_phys op

iova_to_phys is a performance path for the DMA API and iommufd, implement
it using an unrolled get_user_pages() like function waterfall scheme.

The implementation itself is fairly trivial.

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Reviewed-by: Samiullah Khawaja <skhawaja@google.com>
Tested-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
Tested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>