io_uring: consider ring dead once the ref is marked dying

io_uring: consider ring dead once the ref is marked dying

For queueing work to io-wq or adding normal task_work, io_uring will
cancel the work items if the task is going away. If the ring is starting
to go through teardown, the ref is marked as dying. Use that as well for
the fallback/cancel mechanism.

For deferred task_work, this is done out-of-line as part of the exit
work handling. Hence it doesn't need any extra checks in the hot path.

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