LU-16796 lfsck: Change lfsck_layout_slave_target to use kref

LU-16796 lfsck: Change lfsck_layout_slave_target to use kref

This patch changes struct lfsck_layout_slave_target
to use kref instead of atomic_t

Signed-off-by: Arshad Hussain <arshad.hussain@aeoncomputing.com>
Change-Id: I7ea87e2b94a72363971b71415c9430e5b7ded8cc
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/53422
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>