LU-6351 lfsck: check object existence before using it

LU-6351 lfsck: check object existence before using it

Under some cases, when the LFSCK locate the object via its FID,
it does not check whether it exists or not, then further using
such object may access NULL-pointed local object (inode).

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I637eb9bc8c3e283394c8bf9e1250d83659c5e5ba
Reviewed-on: http://review.whamcloud.com/14009
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>