AMD IOMMU: allow disabling only interrupt remapping when certain IVRS consistency...

AMD IOMMU: allow disabling only interrupt remapping when certain IVRS consistency checks fail

After some more thought on the XSA-36 and specifically the comments we
got regarding disabling the IOMMU in this situation altogether making
things worse instead of better, I came to the conclusion that we can
actually restrict the action in affected cases to just disabling
interrupt remapping. That doesn't make the situation worse than prior
to the XSA-36 fixes (where interrupt remapping didn't really protect
domains from one another), but allows at least DMA isolation to still
be utilized.

To do so, disabling of interrupt remapping must be explicitly requested
on the command line - respective checks will then be skipped.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Suravee Suthikulanit <suravee.suthikulpanit@amd.com>