iommufd: pfn_reader for file mappings

iommufd: pfn_reader for file mappings

Extend pfn_reader_user() to pin file mappings, by calling
memfd_pin_folios().  Repin at small page granularity, and fill the batch
from folios.  Expand folios to upages for the iopt_pages_fill() path.

Link: https://patch.msgid.link/r/1729861919-234514-7-git-send-email-steven.sistare@oracle.com
Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>