LU-4972 lfsck: skip .lustre and children for namespace check

LU-4972 lfsck: skip .lustre and children for namespace check

The sub-directories/files under .lustre are either for partial
repaired objects (such as .lustre/lost+found/) or for special
non-real objects (such as .lustre/obf). It is unnecessary to
do namespace LFSCK for them.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I39e8382c226b2ee19a41e6f47af38a5367f3bdd8
Reviewed-on: http://review.whamcloud.com/10143
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>