Fix the disk_io_counter bug in kernel 2.6 to 2.6.25

In kernel from 2.6 to 2.6.25, /proc/diskstats for subpartitions in a disk has only 7 fields.
Check this in http://www.mjmwired.net/kernel/Documentation/iostats.txt
Already test this in kernel 2.6.9, and it works.