iommu/sva: Assign a PASID to mm on PASID allocation and free it on mm exit

PASIDs are process-wide. It was attempted to use refcounted PASIDs to
free them when the last thread drops the refcount. This turned out to
be complex and error prone. Given the fact that the PASID space is 20
bits, which allows up to 1M processes to have a PASID associated
concurrently, PASID resource exhaustion is not a realistic concern.

Therefore, it was decided to simplify the approach and stick with lazy
on demand PASID allocation, but drop the eager free approach and make an
allocated PASID's lifetime bound to the lifetime of the process.

Get rid of the refcounting mechanisms and replace/rename the interfaces
to reflect this new approach.

  [ bp: Massage commit message. ]

Suggested-by: Dave Hansen <[email protected]>
Signed-off-by: Fenghua Yu <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Tony Luck <[email protected]>
Reviewed-by: Lu Baolu <[email protected]>
Reviewed-by: Jacob Pan <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Acked-by: Joerg Roedel <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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