net/mlx5: use indexed pool as id generator

net/mlx5: use indexed pool as id generator

The ID generation API used an integer pool to save released ID, To
support multiple flow, it has to be enhanced to be thread safe.

Indexed pool could be used to generate unique ID by setting size of pool
entry to zero. Since bitmap is used, an extra benefits is saving memory
to about one bit per entry. Further more indexed pool could be thread
safe by enabling lock.

This patch leverages indexed pool to generate ID, removes
unused ID generating API.

Signed-off-by: Xueming Li <xuemingl@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>