iommu/arm-smmu-v3: Make domain_alloc_paging_flags() directly determine the S1/S2

iommu/arm-smmu-v3: Make domain_alloc_paging_flags() directly determine the S1/S2

The selection of S1/S2 is a bit indirect today, make
domain_alloc_paging_flags() directly decode the flags and select the
correct S1/S2 type.

Directly reject flag combinations the HW doesn't support when processing
the flags.

Fix missing rejection of some flag combinations that are not supported
today (ie NEST_PARENT | DIRTY_TRACKING) by using a switch statement to
list out exactly the combinations that are currently supported.

Move the determination of the stage out of arm_smmu_domain_finalise() and
into both callers. As today the default stage is S1 if supported in HW.

This makes arm_smmu_domain_alloc_paging_flags() self contained and no
longer calling arm_smmu_domain_alloc_paging().

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/2-v1-0bb8d5313a27+27b-smmuv3_paging_flags_jgg@nvidia.com
Signed-off-by: Will Deacon <will@kernel.org>