LU-1267 lfsck: rebuild LAST_ID

LU-1267 lfsck: rebuild LAST_ID

The /O/<seq>/LAST_ID records the last oid of the object allocated
within the sequence. The LAST_ID file can be crashed or missed as
the system running. The LFSCK for layout consistency verification
can detect the LAST_ID lost/crashed cases, and can rebuild it via
scanning the whole device.

This functionality is also part of LU-14 live replacement of OST.

Introduce lfsck_notify callback - the LFSCK events notification
channel from the LFSCK to the registered users (MDD/OFD).

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iee85056e2fda1ecba9424c9f0e822643e9f029a8
Reviewed-on: http://review.whamcloud.com/6997
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>