io_uring: get next work with submission ref drop

If after dropping the submission reference req->refs == 1, the request
is done, because this one is for io_put_work() and will be dropped
synchronously shortly after. In this case it's safe to steal a next
work from the request.

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