LU-966 mdd: mdd object may not be exist

LU-966 mdd: mdd object may not be exist

If MDT device has been checked with fsck, some mdd objects
could be removed, so that recovery replay could act on non-existing
objects.

Signed-off-by: Bobi Jam <bobijam@whamcloud.com>
Change-Id: If3a3b72ee70ab2513978ed968c9598ddde11c085
Reviewed-on: http://review.whamcloud.com/1928
Reviewed-by: Fan Yong <yong.fan@whamcloud.com>
Reviewed-by: Mikhail Pershin <tappro@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>