LU-13417 mdd: set default LMV on ROOT

LU-13417 mdd: set default LMV on ROOT

To balance MDT usage, set default LMV on ROOT if it's not set. The
default stripe offset is "-1", and default stripe count is "1". Then
directory created by "mkdir" under ROOT will be scattered on all MDTs
by usage.

Add sanity 0e.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Hongchao Zhang <hongchao@whamcloud.com>
Change-Id: I7a6c752256225b8d065b2c304c4725268df28045
Reviewed-on: https://review.whamcloud.com/38553
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>