LU-10877 lu: fix reference leak

LU-10877 lu: fix reference leak

reference leak fix in dt_locate_at introduced in
f6d6a552398eb1e65857d9bf1afaaf98c8dc1a79.
Use a top level object in debug in lu_object_put to match with
lu_object_get.

Signed-off-by: Alexey Lyashkov <c17817@cray.com>
Change-Id: I6dbeeb4f70a2914b1606ad0c5586db431a6dcd2c
Reviewed-on: https://review.whamcloud.com/31870
Reviewed-by: Andrew Perepechko <c17827@cray.com>
Tested-by: Jenkins
Reviewed-by: Sergey Cheremencev <c17829@cray.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>