io_uring: always grab lock in io_cancel_async_work()

No upstream commit exists for this patch.

It's not necessarily safe to check the task_list locklessly, remove
this micro optimization and always grab task_lock before deeming it
empty.

Reported-and-tested-by: Lee Jones <lee@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>