iommu: Add ops->domain_alloc_nested()

It turns out all the drivers that are using this immediately call into
another function, so just make that function directly into the op. This
makes paging=NULL for domain_alloc_user and we can remove the argument in
the next patch.

The function mirrors the similar op in the viommu that allocates a nested
domain on top of the viommu's nesting parent. This version supports cases
where a viommu is not being used.

Link: https://patch.msgid.link/r/1-v1-c252ebdeb57b+329-iommu_paging_flags_jgg@nvidia.com


Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>