LU-5914 lfsck: dt_index_try before dt_lookup

LU-5914 lfsck: dt_index_try before dt_lookup

Otherwise it may cause dt_lookup() LBUG when locate the parent
directory MDT-object that is not in cache.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ibbed865e58d8f9a4d4b67265b02ba804efb9719e
Reviewed-on: http://review.whamcloud.com/13801
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Cliff White <cliff.white@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>