net/mlx5: Introduce mlx5_flow_steering structure

Instead of having all steering private name spaces and
steering module fields flat in mlx5_core_priv, we wrap
them in mlx5_flow_steering for better modularity and
API exposure.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>