io_uring: wait for cancelations on final ring put

io_uring: wait for cancelations on final ring put

We still offload the cancelation to a workqueue, as not to introduce
dependencies between the exiting task waiting on cleanup, and that
task needing to run task_work to complete the process.

This means that once the final ring put is done, any request that was
inflight and needed cancelation will be done as well. Notably requests
that hold references to files - once the ring fd close is done, we will
have dropped any of those references too.

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