LU-4596 lprocfs: mdt/*/exports/*/uuid is empty after remount

LU-4596 lprocfs: mdt/*/exports/*/uuid is empty after remount

need to assign exp_nid_stats even if old stats are present

Change-Id: I9b0ac4c7ccee6cbd457827b20123bf40d91aaf33
Xyratex-bug-id: MRP-1641
Signed-off-by: Andriy Skulysh <Andriy_Skulysh@xyratex.com>
Reviewed-on: http://review.whamcloud.com/9171
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Emoly Liu <emoly.liu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>