IB/mlx5: Assign DSCP for R-RoCE QPs Address Path

For Routable RoCE QPs, the DSCP should be set in the QP's
address path.

The DSCP's value is derived from the traffic class.

Fixes: 2811ba51 ("IB/mlx5: Add RoCE fields to Address Vector")
Cc: Achiad Shochat <achiad@mellanox.com>
Signed-off-by: Majd Dibbiny <majd@mellanox.com>
Reviewed-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>