LU-2193 ofd: check object exists before orphan destroy

MDS replay object destroys after recovery could meet non-existing
objects, skip it before following futile actions.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: Ibf233a07fa73d3226fdde5e2c020e73f51428f74
Reviewed-on: http://review.whamcloud.com/6266
Tested-by: Hudson
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>