RDMA/rxe: Fix compile warnings on 32-bit

RDMA/rxe: Fix compile warnings on 32-bit

Move the conditional code into a function, with two varients so it is
harder to make these kinds of mistakes.

 drivers/infiniband/sw/rxe/rxe_resp.c: In function 'atomic_write_reply':
 drivers/infiniband/sw/rxe/rxe_resp.c:794:13: error: unused variable 'payload' [-Werror=unused-variable]
   794 |         int payload = payload_size(pkt);
       |             ^~~~~~~
 drivers/infiniband/sw/rxe/rxe_resp.c:793:24: error: unused variable 'mr' [-Werror=unused-variable]
   793 |         struct rxe_mr *mr = qp->resp.mr;
       |                        ^~
 drivers/infiniband/sw/rxe/rxe_resp.c:791:19: error: unused variable 'dst' [-Werror=unused-variable]
   791 |         u64 src, *dst;
       |                   ^~~
 drivers/infiniband/sw/rxe/rxe_resp.c:791:13: error: unused variable 'src' [-Werror=unused-variable]
   791 |         u64 src, *dst;

Fixes: 034e285f8b99 ("RDMA/rxe: Make responder support atomic write on RC service")
Link: https://lore.kernel.org/linux-rdma/Y5s+EVE7eLWQqOwv@nvidia.com/
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>