LU-4615 lfsck: verify index before locating target descriptor

LU-4615 lfsck: verify index before locating target descriptor

Sometimes, the OST index stored in the file layout EA may be crashed,
or maybe the target server has not registered. LFSCK should has the
ability to detect that to avoid invalid accessing.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I83ead7077885b6fc73bc56361725724d087c1957
Reviewed-on: http://review.whamcloud.com/14189
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>