intel-iommu: Fix intel_iommu_unmap_range() with size 0

After some API change, intel_iommu_unmap_range() introduced a assumption that
parameter size != 0, otherwise the dma_pte_clean_range() would have a
overflowed argument. But the user like KVM don't have this assumption before,
then some BUG() triggered.

Fix it by ignoring size = 0.

Signed-off-by: Sheng Yang <sheng@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

drivers/pci/intel-iommu.c

@@ -3600,6 +3600,9 @@ static void intel_iommu_unmap_range(struct iommu_domain *domain,
 {
 	struct dmar_domain *dmar_domain = domain->priv;
 
+	if (!size)
+		return;
+
 	dma_pte_clear_range(dmar_domain, iova >> VTD_PAGE_SHIFT,
 			    (iova + size - 1) >> VTD_PAGE_SHIFT);