iommufd: Have iopt_map_file_pages convert the fd to a file

iommufd: Have iopt_map_file_pages convert the fd to a file

Since dmabuf only has APIs that work on an int fd and not a struct file *,
pass the fd deeper into the call chain so we can use the dmabuf APIs as
is.

Link: https://patch.msgid.link/r/7-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>