iommu: Turn fault_data to iommufd private pointer

iommu: Turn fault_data to iommufd private pointer

A "fault_data" was added exclusively for the iommufd_fault_iopf_handler()
used by IOPF/PRI use cases, along with the attach_handle. Now, the iommufd
version of the sw_msi function will reuse the attach_handle and fault_data
for a non-fault case.

Rename "fault_data" to "iommufd_hwpt" so as not to confine it to a "fault"
case. Move it into a union to be the iommufd private pointer. A following
patch will move the iova_cookie to the union for dma-iommu too after the
iommufd_sw_msi implementation is added.

Since we have two unions now, add some simple comments for readability.

Link: https://patch.msgid.link/r/ee5039503f28a16590916e9eef28b917e2d1607a.1740014950.git.nicolinc@nvidia.com
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>