The workqueue is being used to run deferred work for the android binder.
The "binder_deferred_workqueue" queues only a single work item and hence
does not require ordering. Also, this workqueue is not being used on a
memory recliam path. Hence, the singlethreaded workqueue has been
replaced with the use of system_wq.
System workqueues have been able to handle high level of concurrency
for a long time now and hence it's not required to have a singlethreaded
workqueue just to gain concurrency. Unlike a dedicated per-cpu workqueue
created with create_singlethread_workqueue(), system_wq allows multiple
work items to overlap executions even on the same CPU; however, a
per-cpu workqueue doesn't have any CPU locality or global ordering
guarantee unless the target CPU is explicitly specified and thus the
increase of local concurrency shouldn't make any difference.