io_uring: safer fallback_work free

Add extra wq flushing for fallback_work, that's not necessary but safer
if invariants of io_fallback_req_func() change.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/24179419d6748516299600bc914f50b9e0b02275.1633532552.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>