iommu: Introduce domain attachment handle

Currently, when attaching a domain to a device or its PASID, domain is
stored within the iommu group. It could be retrieved for use during the
window between attachment and detachment.

With new features introduced, there's a need to store more information
than just a domain pointer. This information essentially represents the
association between a domain and a device. For example, the SVA code
already has a custom struct iommu_sva which represents a bond between
sva domain and a PASID of a device. Looking forward, the IOMMUFD needs
a place to store the iommufd_device pointer in the core, so that the
device object ID could be quickly retrieved in the critical fault handling
path.

Introduce domain attachment handle that explicitly represents the
attachment relationship between a domain and a device or its PASID.

Co-developed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20240702063444.105814-2-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>

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