IB/mlx4: Fix handling of wq->tail for send completions

Cast the increment added to wq->tail when send completions are
processed to u16 to avoid using wrong values caused by standard
integer promotions.

The same bug was fixed in libmlx4 by Eli Cohen <eli@mellanox.co.il>.
Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mlx4/cq.c

@@ -354,7 +354,7 @@ static int mlx4_ib_poll_one(struct mlx4_ib_cq *cq,
 	if (is_send) {
 		wq = &(*cur_qp)->sq;
 		wqe_ctr = be16_to_cpu(cqe->wqe_index);
-		wq->tail += wqe_ctr - (u16) wq->tail;
+		wq->tail += (u16) (wqe_ctr - (u16) wq->tail);
 		wc->wr_id = wq->wrid[wq->tail & (wq->max - 1)];
 		++wq->tail;
 	} else if ((*cur_qp)->ibqp.srq) {