Intel-IOMMU Alignment Issue in dma_pte_clear_range()

This issue was pointed out by Linus.

In dma_pte_clear_range() in intel-iommu.c

start = PAGE_ALIGN(start);
end &= PAGE_MASK;
npages = (end - start) / VTD_PAGE_SIZE;

In partial page case, start could be bigger than end and npages will be
negative.

Currently the issue doesn't show up as a real bug in because start and
end have been aligned to page boundary already by all callers. So the
issue has been hidden. But it is dangerous programming practice.

Signed-off-by: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>