ERDMA driver use a workqueue for asynchronous reflush command posting.
Implement the lifecycle of this workqueue.

Link: https://lore.kernel.org/r/20221116023107.82835-2-chengyou@linux.alibaba.comSigned-off-by: NCheng Xu <chengyou@linux.alibaba.com>
Signed-off-by: NJason Gunthorpe <jgg@nvidia.com>