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.