vfio: Do not manipulate iommu dma_owner for fake iommu groups

Since asserting dma ownership now causes the group to have its DMA blocked
the iommu layer requires a working iommu. This means the dma_owner APIs
cannot be used on the fake groups that VFIO creates. Test for this and
avoid calling them.

Otherwise asserting dma ownership will fail for VFIO mdev devices as a
BLOCKING iommu_domain cannot be allocated due to the NULL iommu ops.

Fixes: 0286300e ("iommu: iommu_group_claim_dma_owner() must always assign a domain")
Reported-by: Eric Farman <farman@linux.ibm.com>
Tested-by: Eric Farman <farman@linux.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/0-v1-9cfc47edbcd4+13546-vfio_dma_owner_fix_jgg@nvidia.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>