iommu: Check for deferred attach in iommu_group_do_dma_attach()

The iommu_group_do_dma_attach() must not attach devices which have
deferred_attach set. Otherwise devices could cause IOMMU faults when
re-initialized in a kdump kernel.

Fixes: deac0b3b ("iommu: Split off default domain allocation from group assignment")
Reported-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Jerry Snitselaar <jsnitsel@redhat.com>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Link: https://lore.kernel.org/r/20200604091944.26402-1-joro@8bytes.org