LU-3285 mdt: get lu_object_attr for existent object

LU-3285 mdt: get lu_object_attr for existent object

In mdt_punch_hdl() lu_object_attr() may be taken for
non-existent object with LBUG() as result.
Take lu_object_attr() after mdt_object_exists()check.

Test-Parameters: mdssizegb=20 testlist=sanity-dom,dom-performance
Signed-off-by: Mikhal Pershin <mike.pershin@intel.com>
Change-Id: If1181caa94b28cf1f251f57fa6f7102dc40c329d
Reviewed-on: https://review.whamcloud.com/30057
Tested-by: Jenkins
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>