LU-3528 mdt: check object exists for remote directory

LU-3528 mdt: check object exists for remote directory

Check whether the remote object exists before enqueue and
getattr to avoid LBUG.

Remove unnecssary remote object exist check in mdd_object_lock.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Ia634a8c7b9cd2810515e854163c5fdd6bdf8716f
Reviewed-on: http://review.whamcloud.com/8371
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>