IB/mlx4: Consolidate code to get an entry from a struct mlx4_buf

We use struct mlx4_buf for kernel QP, CQ and SRQ buffers, and the code
to look up an entry is duplicated in get_cqe_from_buf() and the QP and
SRQ versions of get_wqe().  Factor this out into mlx4_buf_offset().

This will also make it easier to switch over to using vmap() for buffers.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

drivers/infiniband/hw/mlx4/qp.c

@@ -96,11 +96,7 @@ static int is_qp0(struct mlx4_ib_dev *dev, struct mlx4_ib_qp *qp)
 
 static void *get_wqe(struct mlx4_ib_qp *qp, int offset)
 {
-	if (qp->buf.nbufs == 1)
-		return qp->buf.u.direct.buf + offset;
-	else
-		return qp->buf.u.page_list[offset >> PAGE_SHIFT].buf +
-			(offset & (PAGE_SIZE - 1));
+	return mlx4_buf_offset(&qp->buf, offset);
 }
 
 static void *get_recv_wqe(struct mlx4_ib_qp *qp, int n)