io_uring: if we see flush on exit, cancel related tasks

Ensure we match tasks that belong to a dead or dying task as well, as we
need to reap those in addition to those belonging to the exiting task.

Cc: stable@vger.kernel.org # 5.9+
Reported-by: Josef Grieb <josef.grieb@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>