net/mlx5: add MPLS to Direct Verbs flow engine

net/mlx5: add MPLS to Direct Verbs flow engine

The support in MPLS on this flow engine was overlooked. It's absence is
critical because there are required actions for MPLS which can be done
only with the DV engine.

To set correctly the MPLS filter, we need to reason about the flow item
before the MPLS (UDP, GRE or other).
To do that, a new variable last_item was added and updated after each
translation. the full item flags are updated after each item iteration.

Signed-off-by: Dekel Peled <dekelp@mellanox.com>
Acked-by: Shahaf Shuler <shahafs@mellanox.com>