dm-latency: add trace info

This patch just adds printk information to trace how code executing.
dm-latency: move struct mapped_device from dm.c to dm.h

Because struct mapped_device is referenced in dm-sysfs.c, move its
definition into dm.h which is included by dm-sysfs.c
dm-latency: add I/O latency measurement in device mapper

This patch adds I/O latency measurement code in dm.c
dm-latency: add sysfs interface

This patch adds sysfs interface to display io latency statistic info