iommufd: File mappings for mdev

Support file mappings for mediated devices, aka mdevs.  Access is
initiated by the vfio_pin_pages() and vfio_dma_rw() kernel interfaces.

Link: https://patch.msgid.link/r/1729861919-234514-9-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>