io_uring: flush completions for fallbacks

io_fallback_req_func() doesn't expect anyone creating inline
completions, and no one currently does that. Teach the function to flush
completions preparing for further changes.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/8b941516921f72e1a64d58932d671736892d7fff.1629286357.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>