Add a responder state for atomic reply similar to read reply and rename
process_atomic() rxe_atomic_reply(). In preparation for merging the normal
and retry atomic responder flows.

Link: https://lore.kernel.org/r/20220606143836.3323-3-rpearsonhpe@gmail.comSigned-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>