The workqueue "user_dlm_worker" queues a single work item
&lockres->l_work per user_lock_res instance and so it doesn't require
execution ordering. Hence, alloc_workqueue has been used to replace the
deprecated create_singlethread_workqueue instance.
The WQ_MEM_RECLAIM flag has been set to ensure forward progress under
memory pressure.
Since there are fixed number of work items, explicit concurrency
limit is unnecessary here.