LU-5285 mdt: reconstruct to handle non-existing objects

LU-5285 mdt: reconstruct to handle non-existing objects

it's possible that reconstruction finds the object doesn't exist
because the original transaction was committed and the rep-ack
lock got released letting another clients to destroy the object.
we should handle this propely - return attributes to indicate the
object has been unlinked (nlink=0

Signed-off-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Change-Id: I3bc18ff0cdb3b7f6d28e47604cf3e142e3f5bc42
Reviewed-on: http://review.whamcloud.com/11025
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>