io_uring: fixup fallback work case

io_uring: fixup fallback work case

For fallback work, we used to grab the io-wq manager task. But now that
they are tied more closely to the actual task, just have a callback list
in the ctx. Then we can run it at exit time, which is the only time that
adding task_work would fail for us.

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