LU-5395 lfsck: misc patch to prevent lfsck hung

1) When the LFSCK rebuilt the crashed LAST_ID files, it will
   notify the MDS to sync lastid information via disconnecting
   the connection. OFD should hold the export reference before
   disconnecting to allow to send RPC reply message.

2) When the layout LFSCK scans on the OST, it needs to handle
   the IDIF objects specially (use fid_idif_id() to get the
   OST object ID) to avoid to regard the LAST_ID file as
   corrupted by wrong.

3) The LFSCK should check the ostid_to_fid() return value for
   corrupted OSTID and/or index.

4) If the LAST_ID file is not crashed, then do not update the
   LAST_ID file.

5) NOT change the lu_buf::lb_len once the lu_buf::lb_buf is
   allocated to prevent accessing released or non-allocated
   RAM space by wrong.

6) Other small fixes and code cleanup.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I84726ddcf0b8fa6b334163fb13d9bae273033d20
Reviewed-on: http://review.whamcloud.com/11304
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>