LU-1866 lfsck: LFSCK for namespace consistency (3)

It is the third part of LFSCK 1.5 for namespace consistency
check and repair. It mainly implements the LFSCK namespace
component functionality lfsck_operations::lfsck_double_scan:
process multiple-linked objects.

1) Find out and repair crashed linkEA entry.

2) Find out and remove invalid linkEA entry

Test-Parameters: envdefinitions=ENABLE_QUOTA=yes testlist=sanity-lfsck

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ia38fa343ed4da10635c1af4d90b9039c5e01688c
Reviewed-on: http://review.whamcloud.com/4914
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>