UPSTREAM: 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.

Bug: 278721720
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>
(cherry picked from commit b0bfceaa)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I112708bdbe43f0e1c2cad5e73a56d7609c10d339