IB/ipoib: Remove deprecated create_singlethread_workqueue

alloc_ordered_workqueue() replaces deprecated
create_singlethread_workqueue().

The workqueue "ipoib_workqueue" that is used for all flush operations
for the device.

WQ_MEM_RECLAIM has been set since the flush operations may need to
complete in order for other network functions to continue, and
the memory reclaim operation might need the network functioning in
order to make progress.

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