LU-15660 statahead: statahead thread doesn't stop

LU-15660 statahead: statahead thread doesn't stop

Add a barrier to ensure sai_task changing can be seen
when access it without locking. Else the statahead
thread could sleep forever since wake_up was lost.

Signed-off-by: Yang Sheng <ys@whamcloud.com>
Change-Id: I211e99f1bdddaaaf028a205658f603fda034d389
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/47673
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>