RDMA/rxe: Add send_common_ack() helper

Most code in send_ack() and send_atomic_ack() are duplicate, move them to
a new helper send_common_ack().

In newer IBA spec, some opcodes require acknowledge with a zero-length
read response, with this new helper, we can easily implement it later.

Link: https://lore.kernel.org/r/1659335010-2-1-git-send-email-lizhijian@fujitsu.com


Signed-off-by: Li Zhijian <lizhijian@fujitsu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>