LU-16135 lod: prohibit DoM pattern in plain layout

LU-16135 lod: prohibit DoM pattern in plain layout

DoM pattern can be set as default directory plain layout by
older LFS version. It misses DoM component sanity checks if
plain layout is used. Such layout is not allowed and causes
later crashed when file is created under that directory.

While LFS can prevent this but not in all Lustre versions,
so LOD should do the check as well

Signed-off-by: Mikhail Pershin <mpershin@whamcloud.com>
Change-Id: Ic58fdda2ab3e63083128cb6cf949fcb43ccd2c02
Reviewed-on: https://review.whamcloud.com/48433
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>