RDMA/rxe: Implement atomic write completion

Generate an atomic write completion when the atomic write request
has been finished.

Link: https://lore.kernel.org/r/1669905568-62-3-git-send-email-yangx.jy@fujitsu.com


Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>