io_uring offloads task_work for cancelation purposes when the task is
exiting. This is conceptually fine, but we should be nicer and actually
wait for that work to complete before returning.

Add an argument to io_fallback_tw() telling it to flush the deferred
work when it's all queued up, and have it flush a ctx behind whenever
the ctx changes.

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