LU-16796 target: Change struct job_stat to use kref

LU-16796 target: Change struct job_stat to use kref

This patch changes struct job_stat to use
kref(refcount_t) instead of atomic_t

Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I02bcf91d4c19915bc37601c90172b1de37b87811
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52799
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>