iommu/vt-d: Add helper to allocate paging domain

iommu/vt-d: Add helper to allocate paging domain

The domain_alloc_user operation is currently implemented by allocating a
paging domain using iommu_domain_alloc(). This is because it needs to fully
initialize the domain before return. Add a helper to do this to avoid using
iommu_domain_alloc().

Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20240610085555.88197-16-baolu.lu@linux.intel.com
Reviewed-by: Yi Liu <yi.l.liu@intel.com>
Link: https://lore.kernel.org/r/20240702130839.108139-6-baolu.lu@linux.intel.com
Signed-off-by: Will Deacon <will@kernel.org>