io_uring: do io_*_prep() early in io_submit_sqe()

Now as preparations are split from async setup, we can do the first one
pretty early not spilling it across multiple call sites. And after it's
done SQE is not needed anymore and we can save on passing it deeply into
the submission stack.

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