net/mlx5_core: Export flow steering API

Add exports to flow steering API for mlx5_ib usage.
The following functions are exported:

1. mlx5_create_auto_grouped_flow_table - used to create flow
table with auto flow grouping management (create and destroy
flow groups). In auto-grouped flow tables, we create groups
automatically if needed (if we don't find an existing
flow group with same match criteria when we add new rule).

2. mlx5_destroy_flow_table - used to destroy  a flow table.

3. mlx5_add_flow_rule - used to add flow rule into a flow table.

4. mlx5_del_flow_rule - used to delete flow rule from its flow table.

5. mlx5_get_flow_namespace - used to get a handle to the required
namespace sub-tree.

Signed-off-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Moni Shoua <monis@mellanox.com>
Signed-off-by: Matan Barak <matanb@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>