net/mlx5: Move metdata fields under offloads structure

Metadata fields are offload mode specific.
To improve code readability, move metadata under offloads structure.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Vu Pham <vuhuong@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>