LU-1866 lfsck: ancillary work for namespace LFSCK

1) Share related linkEA functions with LFSCK.

2) Multiple linked objects are rare, and the objects with
   a lot of linkea entries are more rare. It is unnecessary
   to worry about performance under such cases. So drop the
   linkea entries count limitation: LINKEA_MAX_COUNT.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I75f90c9619c56f2f62063435aea26ed01337d6dd
Reviewed-on: http://review.whamcloud.com/4907
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>