RDMA: Extend RDMA user ABI to support atomic write

1) Define new atomic write request/completion in userspace.
2) Define new atomic write capability in userspace.

Link: https://lore.kernel.org/r/1669905432-14-2-git-send-email-yangx.jy@fujitsu.com


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