Move the allocation of the atomic responder resource up into
rxe_atomic_reply() from send_atomic_ack(). In preparation for merging the
normal and retry atomic responder flows.

Link: https://lore.kernel.org/r/20220606143836.3323-4-rpearsonhpe@gmail.comSigned-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>