vfio: avoid possible overflow in vfio_iommu_type1_pin_pages

add parentheses to avoid possible vaddr overflow.

Fixes: a54eb550

 ("vfio iommu type1: Add support for mediated devices")
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>