io_uring: move all request init code in one place

Requests initialisation is scattered across several functions, namely
io_init_req(), io_submit_sqes(), io_submit_sqe(). Put it
in io_init_req() for better data locality and code clarity.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>