Implement FLUSH request operation in the requester.

Link: https://lore.kernel.org/r/20221206130201.30986-7-lizhijian@fujitsu.comReviewed-by: Zhu Yanjun <zyjzyj2000@gmail.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>