LU-19008 hsm: add locking for coordinator thread stop

LU-19008 hsm: add locking for coordinator thread stop

There is no locking around thread stop, which can race between
mdt_coordinator() and mdt_hsm_cdt_stop() and with use-after-free
during unmount.  Add locking to avoid this.

Fixes: 4512347d6c ("LU-16356 hsm: add running ref to the coordinator")
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I996a79fcbca3b1c6f6a0f5ee5d9f052f31eda61f
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/59425
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>