vdpa/mlx5: add task ring for multi-thread management

vdpa/mlx5: add task ring for multi-thread management

The configuration threads tasks need a container to
support multiple tasks assigned to a thread in parallel.
Use rte_ring container per thread to manage
the thread tasks without locks.
The caller thread from the user context opens a task to
a thread and enqueue it to the thread ring.
The thread polls its ring and dequeue tasks.
That’s why the ring should be in multi-producer
and single consumer mode.
Anatomic counter manages the tasks completion notification.
The threads report errors to the caller by
a dedicated error counter per task.

Signed-off-by: Li Zhang <lizh@nvidia.com>
Acked-by: Matan Azrad <matan@nvidia.com>
Reviewed-by: Maxime Coquelin <maxime.coquelin@redhat.com>