LU-4106 mdt: avoid recursive lu_object_find an object

LU-4106 mdt: avoid recursive lu_object_find an object

LU-3240 (commit 762f2114d282a98ebfa4dbbeea9298a8088ad24e) set parent
dir fid the same as child fid in getattr by fid case we should not
lu_object_find() the object again, could lead to hung if there is a
concurrent unlink destroyed the object.

Signed-off-by: Bobi Jam <bobijam.xu@intel.com>
Change-Id: I75256c0fa684877cbd2e1f36f8ab2ac3faab2989
Reviewed-on: http://review.whamcloud.com/7990
Tested-by: Jenkins
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>