vfio/type1: Fix unmap overflow off-by-one

The below referenced commit adds a test for integer overflow, but in
doing so prevents the unmap ioctl from ever including the last page of
the address space.  Subtract one to compare to the last address of the
unmap to avoid the overflow and wrap-around.

Fixes: 71a7d3d7 ("vfio/type1: silence integer overflow warning")
Link: https://bugzilla.redhat.com/show_bug.cgi?id=1662291
Cc: stable@vger.kernel.org # v4.15+
Reported-by: Pei Zhang <pezhang@redhat.com>
Debugged-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Tested-by: Peter Xu <peterx@redhat.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_iommu_type1.c

@@ -878,7 +878,7 @@ 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 ||
+	if (unmap->iova + unmap->size - 1 < unmap->iova ||
 	    unmap->size > SIZE_MAX)
 		return -EINVAL;