iommu: flush I/O TLB if iommu_map() or iommu_unmap() fail

iommu: flush I/O TLB if iommu_map() or iommu_unmap() fail

This patch adds a full I/O TLB flush to the error paths of iommu_map() and
iommu_unmap().

Without this change callers need constructs such as:

rc = iommu_map/unmap(...)
err = iommu_flush(...)
if ( !rc )
  rc = err;

With this change, it can be simplified to:

rc = iommu_map/unmap(...)
if ( !rc )
  rc = iommu_flush(...)

because, if the map or unmap fails the flush will be unnecessary. This saves
a stack variable and generally makes the call sites tidier.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>