io_uring: don't take ctx refs in tctx_task_work()

io_uring: don't take ctx refs in tctx_task_work()

Now we ban any new req-task_works to be added after we start the task
cancellation. Because tctx is removed from ctx lists only during
task cancellation, and considering that it's removed from the task
context, we'll have current accounted in all rings tctx_task_work() is
working with and so they will stay alive at least awhile it's running.
Don't takes extra ctx refs.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/92eb2d1f934ad16752cc3b764d8be75ca04d1ffe.1650548192.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>