net/mlx5: DR, Manage STE send info objects in pool

Instead of allocating/freeing send info objects dynamically, manage them
in pool. The number of send info objects doesn't depend on rules, so after
pre-populating the pool with an initial batch of send info objects, the
pool is not expected to grow.
This way we save alloc/free during writing STEs to ICM, which can
sometimes take up to 40msec.

Signed-off-by: Yevgeny Kliteynik <kliteyn@nvidia.com>
Reviewed-by: Alex Vesker <valex@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>