RDMA/rxe: Move code to rxe_prepare_atomic_res()

Separate the code that prepares the atomic responder resource into a
subroutine. This is preparation for merging the normal and retry atomic
responder flows.

Link: https://lore.kernel.org/r/20220606143836.3323-2-rpearsonhpe@gmail.com


Signed-off-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>