It seems that the occasional data corruption observed with the tg3
driver wasn't due to missing barriers after all, but rather seems to
be due to the DART (= IOMMU) in the U4 northbridge reading stale
IOMMU table entries from memory due to a race.  This fixes it by
making the CPU read the entry back from memory before using it.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>