intel-iommu: intel_iommu_map_range failed at very end of address space

intel_iommu_map_range() doesn't allow allocation at the very end of the
address space; that code has been simplified and corrected.

Signed-off-by: Tom Lyon <pugs@cisco.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>