md: Fix stray --cluster-confirm crash

A --cluster-confirm without an --add (by another node) can
crash the kernel.

Fix it by guarding it using a state.

Signed-off-by: Goldwyn Rodrigues <rgoldwyn@suse.com>
Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/md-cluster.h b/drivers/md/md-cluster.h
index 60d7e58..7417133 100644
--- a/drivers/md/md-cluster.h
+++ b/drivers/md/md-cluster.h
@@ -21,7 +21,7 @@
 	int (*area_resyncing)(struct mddev *mddev, sector_t lo, sector_t hi);
 	int (*add_new_disk_start)(struct mddev *mddev, struct md_rdev *rdev);
 	int (*add_new_disk_finish)(struct mddev *mddev);
-	void (*new_disk_ack)(struct mddev *mddev, bool ack);
+	int (*new_disk_ack)(struct mddev *mddev, bool ack);
 };
 
 #endif /* _MD_CLUSTER_H */