LU-1546 mdd: return nlink as zero for dead objects

In mdd_attr_get_internal() check for dead objects and set nlink to
zero accordingly.  Add a sanity check to verify that this works as
expected.

Change-Id: I9dd574cce63bcbf47c9f8f3c7ae6d151c704a45a
Signed-off-by: John L. Hammond <john.hammond@intel.com>
Reviewed-on: http://review.whamcloud.com/3154
Tested-by: Hudson
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>