net/mlx5: optimize shared counter memory

net/mlx5: optimize shared counter memory

Instead of using special memory to indicate shared counter, this patch
does the optimization to use the counter handler reserved memory to
indicate it.  The counter index with MLX5_CNT_SHARED_OFFSET means the
shared counter.

This patch is also an arrangement for a new adjustment to use batch
counter as shared counter.

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