dm-latency: provide is_dm_target_latency_ok() by lib_dm_io_latency.c

lib_dm_io_latency.c provides is_dm_target_latency_ok() api to check
sysfs interface, if the latency stats reaches the pre-defined threshold,
is_dm_target_latency_ok() returns 0, otherwise return 1.

By this api, application (most of time it's hard disk health monitor)
may know whether an action should be triggered.
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.
dm-latency: rebase patches to upstream kernel

This commit rebases patches to upstream kernel,
    0003-dm-latency-add-latency-measurement.patch
    0004-dm-latency-add-sysfs-interface.patch