net/mlx5: optimize shared RSS action memory

net/mlx5: optimize shared RSS action memory

The RSS shared action was saved in flow memory by a pointer.
It means that every flow memory includes 8B only for optional shared
RSS case.

Move the RSS objects to be used by indexed pool which reduces the flow
handle memory to 4B.

So, now, the shared action handler is also just a 4B index.

Signed-off-by: Matan Azrad <matan@nvidia.com>
Acked-by: Dekel Peled <dekelp@nvidia.com>