iommufd: Consistently manage hwpt_item

This should be added immediately after every iopt_table_add_domain(), and
deleted after every iopt_table_remove_domain() under the ioas->mutex.

Tidy things to be consistent.

Link: https://lore.kernel.org/r/3-v3-ae9c2975a131+2e1e8-iommufd_hwpt_jgg@nvidia.com


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