Implement support for Differentiated Service Code Point, DSCP, in mlx5en(4).

The DSCP feature is controlled using a set of sysctl(8) fields under
the qos sysctl directory entry for mlx5en(4).

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.

Linux commit:
ed88451e1f2d400fd6a743d0a481631cf9f97550

MFC after: 1 week
Sponsored by: Mellanox Technologies