dm-latency: bug fix in dm_attr_io_latency_ms_show()

Previous commit missing returning strlen(buf) at the end of
dm_attr_io_latency_ms_show(). This commit fixes this bug.
dm-latency: re-compile binary dm-mod for fixed code

This commit add the re-compiled binary dm-mod, which is build from the
code in last commit. Just for convenience.