LU-2915 lfsck: LFSCK 1.5 technical debts (3)

LU-2915 lfsck: LFSCK 1.5 technical debts (3)

This patch resolves some LFSCK 1.5 technical debts, including:
1) Check and remove repeated linkea entries.
2) Merge some "goto" branches to make the code more readable.
3) Some comments about object's nlink inconsistency processing.

Test-Parameters: testlist=sanity-scrub,sanity-lfsck
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Ia2bf525cab6b94b87b7d88133393f9d2bb13031e
Reviewed-on: http://review.whamcloud.com/6344
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>