LU-8994 lmv: honour the specified stripe index

LU-8994 lmv: honour the specified stripe index

when creating the striped directory, specified
stripe index should always be used even the parent
has default stripe index.

Signed-off-by: Di Wang <di.wang@intel.com>
Change-Id: Ief00af3790282a0887b9d28c581ccf0070cc758f
Reviewed-on: https://review.whamcloud.com/24777
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Jenkins
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>