net/mlx5: optimize free counter lookup

net/mlx5: optimize free counter lookup

Currently, when allocate a new counter, it needs loop the whole
container pool list to get a free counter.

In the case with millions of counters allocated, and all the pools
are empty, allocate the new counter will still need to loop the
whole container pool list first, then allocate a new pool to get a
free counter. It wastes the cycles during the pool list traversal.

Add a global free counter list in the container helps to get the free
counters more efficiently.

Signed-off-by: Suanming Mou <suanmingm@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>