LU-5511 lfsck: repair unmatched parent-child pairs

LU-5511 lfsck: repair unmatched parent-child pairs

If the parent directry_A reference the child directory_B via the
name entry_C, but the child directory_B back references another
parent directory_D via its ".." name entry, then the namespace
LFSCK should can find out such inconsistency in the second-stage
scanning and repair it by trusting the name entry_B basically.

Usually, for local filesystem or Lustre backend system, such as
ldiskfs, the local filesystem consistency verification tools,
such as e2fsck can guarantee such consistency. So the namespace
LFSCK will mainly focus on the corss-MDTs parent-child pairs
consistency verification.

Lustre does not support hardlink on directory object. So if the
directory contains multiple linkEA entries or bad linkEA entry,
then the namespace LFSCK should can find out and repair related
inconsistency.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: If8cb02f76329fe04fe3a6c280e6926d014654322
Reviewed-on: http://review.whamcloud.com/11486
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>