blk-throttle: Avoid tracking latency if low limit is invalid

The IO latency tracking is only for LOW limit, so we should add a
validation to avoid redundant latency tracking if the LOW limit
is not valid.

Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>