LU-4725 obd: lu_object_find_at hung

LU-4725 obd: lu_object_find_at hung

lu_object_find_at hungs if called 2 times and object is removed
in between. It makes mdt_rename_sanity not workable for rename.
Change mdt_rename_sanity to work on existing object.

Signed-off-by: Vitaly Fertman <vitaly_fertman@xyratex.com>
Xyratex-bug-id: MRP-1700
Change-Id: I173e2fe3f87bc88fcc73c1813cfe4298fb515c50
Reviewed-on: http://review.whamcloud.com/10484
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: wangdi <di.wang@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Tested-by: John L. Hammond <john.hammond@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>