dm-latency: rebase patches by redhat 2.6.32 kernel
dm-latency: add reset function to dm-latency in sysfs interface

Now echo any thing into /sys/block/dm-X/dm/io_latency_reset may reset
all counters in /sys/block/dm-X/dm/io_latency_{ms, us} to 0.

This function might be helpful to application developers.