{net,IB}/mlx5: Add has_tag to mlx5_flow_act

The has_tag member will indicate whether a tag action was specified
in flow specification.

A flow tag 0 = MLX5_FS_DEFAULT_FLOW_TAG is assumed a valid flow tag
that is currently used by mlx5 RDMA driver, whereas in HW flow_tag = 0
means that the user doesn't care about flow_tag.  HW always provide
a flow_tag = 0 if all flow tags requested on a specific flow are 0.

So we need a way (in the driver) to differentiate between a user really
requesting flow_tag = 0 and a user who does not care, in order to be
able to report conflicting flow tags on a specific flow.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Aviad Yehezkel <aviadye@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>