iommu: Sort out domain user data

When DMA/MSI cookies were made first-class citizens back in commit
46983fcd67ac ("iommu: Pull IOVA cookie management into the core"), there
was no real need to further expose the two different cookie types.
However, now that IOMMUFD wants to add a third type of MSI-mapping
cookie, we do have a nicely compelling reason to properly dismabiguate
things at the domain level beyond just vaguely guessing from the domain
type.

Meanwhile, we also effectively have another "cookie" in the form of the
anonymous union for other user data, which isn't much better in terms of
being vague and unenforced. The fact is that all these cookie types are
mutually exclusive, in the sense that combining them makes zero sense
and/or would be catastrophic (iommu_set_fault_handler() on an SVA
domain, anyone?) - the only combination which *might* be reasonable is
perhaps a fault handler and an MSI cookie, but nobody's doing that at
the moment, so let's rule it out as well for the sake of being clear and
robust. To that end, we pull DMA and MSI cookies apart a little more,
mostly to clear up the ambiguity at domain teardown, then for clarity
(and to save a little space), move them into the union, whose ownership
we can then properly describe and enforce entirely unambiguously.

[nicolinc: rebase on latest tree; use prefix IOMMU_COOKIE_; merge unions
           in iommu_domain; add IOMMU_COOKIE_IOMMUFD for iommufd_hwpt]

Link: https://patch.msgid.link/r/1ace9076c95204bbe193ee77499d395f15f44b23.1742871535.git.nicolinc@nvidia.com
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

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