LU-16796 target: Change struct barrier_instance to use kref

LU-16796 target: Change struct barrier_instance to use kref

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

Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I2d2312b64d873e58bbef449c0867c679feb0c31b
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52173
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>