iommufd: Add iommufd_lock_obj() around the auto-domains hwpts

A later patch will require this locking - currently under the ioas mutex
the hwpt can not have a 0 reference and be on the list.

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


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