io_uring: init opcode in io_init_req()

Move io_req_prep() call inside of io_init_req(), it simplifies a bit
error handling for callers.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/a0f59291fd52da4672c323542fd56fd899e23f8f.1633107393.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>