LU-13709 utils: 'lfs mkdir -i -1' doesn't work

LU-13709 utils: 'lfs mkdir -i -1' doesn't work

'lfs mkdir -i -1 -c...' is to create directory on MDTs by space usage,
when stripe count is more than 1, the target MDT list is not correctly
initialized, which will cause command fail.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Id4584940cec390a9245e888c96c7873f5afa209e
Reviewed-on: https://review.whamcloud.com/39165
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>