AMD/IOMMU: Common the #732/#733 errata handling in iommu_read_log()

AMD/IOMMU: Common the #732/#733 errata handling in iommu_read_log()

There is no need to have both helpers implement the same workaround.  The size
and layout of the the Event and PPR logs (and others for that matter) share a
lot of commonality.

Use MASK_EXTR() to locate the code field, and use ACCESS_ONCE() rather than
barrier() to prevent hoisting of the repeated read.

Avoid unnecessary zeroing by only clobbering the 'code' field - this alone is
sufficient to spot the errata when the rings wrap.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>