io_uring: drop 'ctx' ref on task work cancelation

If task_work ends up being marked for cancelation, we go through a
cancelation helper instead of the queue path. In converting task_work to
always hold a ctx reference, this path was missed. Make sure that
io_req_task_cancel() puts the reference that is being held against the
ctx.

Fixes: 6d816e08 ("io_uring: hold 'ctx' reference around task_work queue + execute")
Signed-off-by: Jens Axboe <axboe@kernel.dk>