iommufd: Move iommufd_device to iommufd_private.h

hw_pagetable.c will need this in the next patches.

Link: https://lore.kernel.org/r/5-v3-ae9c2975a131+2e1e8-iommufd_hwpt_jgg@nvidia.com


Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>