alloc_workqueue replaces deprecated create_workqueue().

A dedicated workqueue has been used since the workitem viz
(&lio->txq_status_wq.wk.work which maps to octnet_poll_check_txq_status)
is involved in a brief poll routine for checking transmit queue status
and is an intergral part of normal device operation.
WQ_MEM_RECLAIM has been set to guarantee forward progress under memory
pressure, which is a requirement here.
Since there are only a fixed number of work items, explicit concurrency
limit is unnecessary.

flush_workqueue is unnecessary since destroy_workqueue() itself calls
drain_workqueue() which flushes repeatedly till the workqueue
becomes empty.
Signed-off-by: Bhaktipriya Shridhar <bhaktipriya96@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>