net/mlx5: reorganize flow structures

net/mlx5: reorganize flow structures

Common structures used for mlx5 flow creating and destroying are
reorganized in order to separating the parts only for destroying
from all the items.
The "mlx5_flow" will contain the common items of DV and Verbs flow,
specific items for DV / Verbs only. These items will only be used
when creating a flow.
At the end of "mlx5_flow", a nested structure "mlx5_flow_handle"
located. It contains all the items used both for creating and
destroying a flow. Also, it consists of common items, and DV / Verbs
specific items.

Signed-off-by: Bing Zhao <bingz@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>