LU-2785 mdd: do not use mti_fid in mdd_find_or_create_root()

as in the subsequent callchain it can be used by
mdd_create_sanity_check()

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: Ia357f587192c54e3c406a35054dca47580077056
Reviewed-on: http://review.whamcloud.com/5311
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>