dm-latency: add stats grainess in second unit

There is feed back from online testing, I/O latencies beyond 500ms are
easy to be observed. Therefore I change the max measurable latency from
500ms to 1000s.

This commit changes patches including,
 0003-dm-latency-add-latency-measurement.patch
 0004-dm-latency-add-sysfs-interface.patch
 0005-dm-latency-add-reset-function-in-sysfs-interface.patch

The changes include,
1) add latency_stats_s[] in struct mapped_device, record I/O latency in
second unit.
2) add sysfs file /sys/block/dm-X/dm/io_latency_s to display I/O latency
statistic in second unit.
3) change max measurable latency from 490ms to 999s.