iommufd: Have pfn_reader process DMABUF iopt_pages

iommufd: Have pfn_reader process DMABUF iopt_pages

Make another sub implementation of pfn_reader for DMABUF. This version
will fill the batch using the struct phys_vec recorded during the
attachment.

Link: https://patch.msgid.link/r/6-v2-b2c110338e3f+5c2-iommufd_dmabuf_jgg@nvidia.com
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Tested-by: Shuai Xue <xueshuai@linux.alibaba.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>