io_uring: don't reuse linked_timeout

Clear linked_timeout for next requests in __io_queue_sqe() so we won't
queue it up unnecessary when it's going to be punted.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Cc: stable@vger.kernel.org # v5.9
Signed-off-by: Jens Axboe <axboe@kernel.dk>