iommu: Do not call domain_alloc() in iommu_sva_domain_alloc()

No driver implements SVA under domain_alloc() anymore, this is dead
code.

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Link: https://lore.kernel.org/r/4-v4-ff5fb6b03bd1+288-iommu_virtio_domains_jgg@nvidia.com


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