RDMA/rxe: Add common rxe_prepare_res()

RDMA/rxe: Add common rxe_prepare_res()

It's redundant to prepare resources for Read and Atomic
requests by different functions. Replace them by a common
rxe_prepare_res() with different parameters. In addition,
the common rxe_prepare_res() can also be used by new Flush
and Atomic Write requests in the future.

Link: https://lore.kernel.org/r/20220705145212.12014-1-yangx.jy@fujitsu.com
Signed-off-by: Xiao Yang <yangx.jy@fujitsu.com>
Reviewed-by: Bob Pearson <rpearsonhpe@gmail.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>