io_uring: remove work flags after cleanup

Shouldn't be a problem now, but it's better to clean
REQ_F_WORK_INITIALIZED and work->flags only after relevant resources are
killed, so cancellation see them.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>