IOLimitTuner may not work well in a special case

[JinheLin]

If both max_bg_read_bytes_per_sec and max_fg_read_bytes_per_sec are less than io_config.min_bytes_per_sec and bg_read runs out of the bandwidth quota, but fg_read has not been used.

The usage rate of read is (max_bg_read_bytes_per_sec + 0 ) / (max_bg_read_bytes_per_sec + max_fg_read_bytes_per_sec), about 50%. It is less than io_config.medium_pct (60% by default), so watermark is LOW.

The LOW watermark means that bandwidth quota of read is sufficient since the usage rate is less than 60%, so it is unnessary to increase its bandwidth quota by decreasing the bandwidth quota of write.

In this case, bg_read will increase its bandwidth quota by decreasing the bandwidth quota of fg_read. However, fg_read is less than io_config.min_bytes_per_sec, it is too small to decrease.