block: don't show io_timeout if driver has no timeout handler

If the low level driver has no timeout handler, the
/sys/block/<disk>/queue/io_timeout will not be displayed.

Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Weiping Zhang <zhangweiping@didiglobal.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>