iommufd: Do not add the same hwpt to the ioas->hwpt_list twice

In the Linux kernel, the following vulnerability has been resolved: iommufd: Do not add the same hwpt to the ioas->hwpt_list twice The hwpt is added to the hwpt_list only during its creation, it is never added again. This hunk is some missed leftover from rework. Adding it twice will corrupt the linked list in some cases. It effects HWPT specific attachment, which is something the test suite cannot cover until we can create a legitimate struct device with a non-system iommu "driver" (ie we need the bus removed from the iommu code)