IB/mad: Remove deprecated create_singlethread_workqueue

The workqueue "ib_nl" queues work items &ib_nl_timed_work and
&mad_agent_priv->local_work. It has been identity converted.

WQ_MEM_RECLAIM has been set to ensure forward progress under
memory pressure.

Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>