net/mlx5: fs, expose flow table ID to users

Expose the flow table ID to users. This will be used by downstream
patches to allow creating steering rules that point to a flow table ID.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>