RDMA: Extend RDMA user ABI to support flush

This commit extends the RDMA user ABI to support the flush
operation defined in IBA A19.4.1. These changes are
backward compatible with the existing RDMA user ABI.

Link: https://lore.kernel.org/r/20221206130201.30986-2-lizhijian@fujitsu.com


Reviewed-by: Zhu Yanjun <zyjzyj2000@gmail.com>
Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>