IB/hfi1: Use QPN mask to avoid overflow

Ensure we can't come up with an array size that is bigger than the array
by applying the QPN mask before the divide in the free_qpn function.
Reviewed-by: Michael J. Ruhl <michael.j.ruhl@intel.com>
Signed-off-by: Dennis Dalessandro <dennis.dalessandro@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>

drivers/infiniband/sw/rdmavt/qp.c

@@ -645,7 +645,7 @@ static void rvt_free_qpn(struct rvt_qpn_table *qpt, u32 qpn)
 {
 	struct rvt_qpn_map *map;
 
-	map = qpt->map + qpn / RVT_BITS_PER_PAGE;
+	map = qpt->map + (qpn & RVT_QPN_MASK) / RVT_BITS_PER_PAGE;
 	if (map->page)
 		clear_bit(qpn & RVT_BITS_PER_PAGE_MASK, map->page);
 }