LU-7988 hsm: mark the cdt as stopped when its thread exits

LU-7988 hsm: mark the cdt as stopped when its thread exits

Use kthread_stop() to stop and join the coordinator thread. Only after
that step can the coordinator state be set to CDT_STOPPED. As a
coordinator doesn't stop instantly, this closes a race if the
coordinator is being restarted at the same time, leaving one thread
shutting down and a new one starting up.

Test-Parameters: trivial testlist=sanity-hsm
Signed-off-by: frank zago <fzago@cray.com>
Change-Id: I0a21d0d22403a56a8965441e1b57118073b6f210
Signed-off-by: Ben Evans <bevans@cray.com>
Reviewed-on: https://review.whamcloud.com/22762
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Quentin Bouget <quentin.bouget@cea.fr>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>