io_uring: consider ring dead once the ref is marked dying

io_uring: consider ring dead once the ref is marked dying

Don't gate this on the task exiting flag. It's generally not a good idea
to gate it on the task PF_EXITING flag anyway. Once the ring is starting
to go through ring teardown, the ref is marked as dying. Use that as
our fallback/cancel mechanism.

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