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The nested domain fields are exclusive to those that used for a DMA
remapping domain. Use union to avoid memory waste.

Link: https://lore.kernel.org/r/20231026044216.64964-3-yi.l.liu@intel.com

Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Yi Liu <yi.l.liu@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>