RDMA/rxe: Extend rxe user ABI to support flush

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

The user API request a flush by filling this structure.

Link: https://lore.kernel.org/r/20221206130201.30986-4-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>