[PATCH] ppc64: More IOMMU cleanups

[PATCH] ppc64: More IOMMU cleanups

* Tidy up some of the arguments to iommu_*()
* Comment cleanup
* Don't bump the hint to the next block for large allocs, to avoid
  fragmentation.
* Simplify vmerge logic during SG allocations
* Move the memory barriers from the bus-specific parts into the common
  code.

Some changes are mine, some are from Ben Herrenschmidt.