LU-4923 osd-ldiskfs: dirdata is not needed on MGS

Don't print a warning message if the "dirdata" feature is not enabled
on MGS devices.  It is only needed for ldiskfs MDT devices.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: I1d8ccfc9c60eff128b480ea8efa298c1212c041a
Reviewed-on: https://review.whamcloud.com/29274


Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>