staging: lustre: 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>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-6373
Reviewed-on: http://review.whamcloud.com/14096


Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>