vfio/type1: silence integer overflow warning

I get a static checker warning about the potential integer overflow if
we add "unmap->iova + unmap->size".  The integer overflow isn't really
harmful, but we may as well fix it.  Also unmap->size gets truncated to
size_t when we pass it to vfio_find_dma() so we could check for too high
values of that as well.
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_iommu_type1.c

@@ -767,6 +767,9 @@ static int vfio_dma_do_unmap(struct vfio_iommu *iommu,
 		return -EINVAL;
 	if (!unmap->size || unmap->size & mask)
 		return -EINVAL;
+	if (unmap->iova + unmap->size < unmap->iova ||
+	    unmap->size > SIZE_MAX)
+		return -EINVAL;
 
 	WARN_ON(mask & PAGE_MASK);
 again: