LU-10224 obd: free obd_svc_stats when all users are gone

LU-10224 obd: free obd_svc_stats when all users are gone

During object device shutdown obd_svc_stats must only be freed
after all access methods from user-land are no longer possible
to prevent any race and further crash.

Signed-off-by: Bruno Faccini <bruno.faccini@intel.com>
Change-Id: Iea4f5b0486779c1721c90f32538af1a723f76a79
Reviewed-on: https://review.whamcloud.com/30249
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Tested-by: Oleg Drokin <oleg.drokin@intel.com>