iommufd/device: Wrap igroup->hwpt and igroup->device_list into attach struct

iommufd/device: Wrap igroup->hwpt and igroup->device_list into attach struct

The igroup->hwpt and igroup->device_list are used to track the hwpt attach
of a group in the RID path. While the coming PASID path also needs such
tracking. To be prepared, wrap igroup->hwpt and igroup->device_list into
attach struct which is allocated per attaching the first device of the
group and freed per detaching the last device of the group.

Link: https://patch.msgid.link/r/20250321171940.7213-8-yi.l.liu@intel.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>