net/mlx5: Support encap id when setting new steering entry

In order to support steering rules which add encapsulation headers,
encap_id parameter is needed.

Add new mlx5_flow_act struct which holds action related parameter:
action, flow_tag and encap_id. Use mlx5_flow_act struct when adding a new
steering rule.
This patch doesn't change any functionality.

Signed-off-by: Hadar Hen Zion <hadarh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>