blk-throttle: force user to configure all settings for io.low

Default value of io.low limit is 0. If user doesn't configure the limit,
last patch makes cgroup be throttled to very tiny bps/iops, which could
stall the system. A cgroup with default settings of io.low limit really
means nothing, so we force user to configure all settings, otherwise
io.low limit doesn't take effect. With this stragety, default setting of
latency/idle isn't important, so just set them to very conservative and
safe value.

Signed-off-by: Shaohua Li <shli@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>