io_uring/rw: don't lose short results on io_setup_async_rw()

If a retry io_setup_async_rw() fails we lose result from the first
io_iter_do_read(), which is a problem mostly for streams/sockets.

Cc: stable@vger.kernel.org
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/0e8d20cebe5fc9c96ed268463c394237daabc384.1664235732.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>