iommu: Create __iommu_alloc_identity_domain()

Consolidate all the code to create an IDENTITY domain into one
function. This removes the legacy __iommu_domain_alloc() path from all
paths, and preps it for final removal.

BLOCKED/IDENTITY/PAGING are now always allocated via a type specific
function.

[Joerg: Actually remove __iommu_domain_alloc()]

Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/20241028093810.5901-13-vasant.hegde@amd.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>