LU-6373 llite: default dir stripe index only for mkdir

LU-6373 llite: default dir stripe index only for mkdir

Default dir stripe index should only work during mkdir,
otherwise it will cause other open/create request being
sent to the wrong MDT.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Id95d7218196d52950eceea38be5612ffe4a6b080
Reviewed-on: http://review.whamcloud.com/14096
Tested-by: Jenkins
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>