[PATCH] powerpc: 64k pages vs. U3 iommu

That DART (U3 iommu) code didn't properly scale the number of entries
when using !4k pages. That caused crashes when booting G5s with more
than 2Gb of RAM. This fixes it.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>