[multipathd] shut a libdevmapper error message when adding paths

[multipathd] shut a libdevmapper error message when adding paths

"device-mapper ioctl cmd 12 failed: No such device or address" was
reported upon path add because we tried to submit a device-mapper ioctl
before the map is ready to receive it (and loop till it works or timeout).

shut the libdevmapper for this case with dm_init_log(&dummy_log) during
this operation.