LU-16796 lfsck: Change struct lfsck_tgt_desc to use kref

LU-16796 lfsck: Change struct lfsck_tgt_desc to use kref

This patch changes struct lfsck_tgt_desc to use
kref instead of atomic_t

Test-Parameters: trivial testlist=sanity-lfsck
Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I1f9bf1889fcc6f0a4b4050c1a2be98476c5254dd
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56141
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>