io_uring/rw: move buffer_select outside generic prep

Cleans up the generic rw prep to not require the do_import flag. Use a
different prep function for callers that might need buffer select.

Based-on-a-patch-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20250227223916.143006-2-kbusch@meta.com


Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>