LU-7462 mdd: check object existence

LU-7462 mdd: check object existence

Check object existence in mdd_is_parent()
and _mdd_lookup(), so the following retrieving
attributes will not panic if object does not
exist.

Signed-off-by: Di Wang <di.wang@intel.com>
Change-Id: Icae243949e202f9d6a4d38b9823373101a093c74
Reviewed-on: http://review.whamcloud.com/17323
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Andreas Dilger <andreas.dilger@intel.com>