LU-5892 lfsck: remove improper LASSERT in lfsck_needs_scan_dir

LU-5892 lfsck: remove improper LASSERT in lfsck_needs_scan_dir

Inside the lfsck_needs_scan_dir(), when the internal variable @fid
becomes the input @obj's parent FID, the internal variable @depth
may be still zero, so the original "LASSERT(depth > 0);" is improper
under such case. Then remove it.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I64f10be682c51c6ac5cc1af3497eb569281fcd21
Reviewed-on: http://review.whamcloud.com/12670
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>