io_uring: run fallback on cancellation

io_uring_try_cancel_requests() matches not only current's requests, but
also of other exiting tasks, so we need to actively cancel them and not
just wait, especially since the function can be called on flush during
do_exit() -> exit_files().
Even if it's not a problem for now, it's much nicer to know that the
function tries to cancel everything it can.

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