LU-16796 lfsck: Change lfsck_assistant_object to use kref

LU-16796 lfsck: Change lfsck_assistant_object to use kref

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

Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I763a44d2c74f758da5a137c6673f8dfd2ef6dc0a
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/52811
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>