md-cluster: Constify struct md_cluster_operations

'struct md_cluster_operations' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  51941	   1442	     80	  53463	   d0d7	drivers/md/md-cluster.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  52133	   1246	     80	  53459	   d0d3	drivers/md/md-cluster.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/3727f3ce9693cae4e62ae6778ea13971df805479.1719173852.git.christophe.jaillet@wanadoo.fr

drivers/md/md-cluster.c

@@ -1570,7 +1570,7 @@ static int gather_bitmaps(struct md_rdev *rdev)
 	return err;
 }
 
-static struct md_cluster_operations cluster_ops = {
+static const struct md_cluster_operations cluster_ops = {
 	.join   = join,
 	.leave  = leave,
 	.slot_number = slot_number,

drivers/md/md.c

@@ -85,7 +85,7 @@ static DEFINE_SPINLOCK(pers_lock);
 
 static const struct kobj_type md_ktype;
 
-struct md_cluster_operations *md_cluster_ops;
+const struct md_cluster_operations *md_cluster_ops;
 EXPORT_SYMBOL(md_cluster_ops);
 static struct module *md_cluster_mod;
 
@@ -8543,7 +8543,7 @@ int unregister_md_personality(struct md_personality *p)
 }
 EXPORT_SYMBOL(unregister_md_personality);
 
-int register_md_cluster_operations(struct md_cluster_operations *ops,
+int register_md_cluster_operations(const struct md_cluster_operations *ops,
 				   struct module *module)
 {
 	int ret = 0;

drivers/md/md.h

@@ -849,7 +849,7 @@ static inline void safe_put_page(struct page *p)
 
 extern int register_md_personality(struct md_personality *p);
 extern int unregister_md_personality(struct md_personality *p);
-extern int register_md_cluster_operations(struct md_cluster_operations *ops,
+extern int register_md_cluster_operations(const struct md_cluster_operations *ops,
 		struct module *module);
 extern int unregister_md_cluster_operations(void);
 extern int md_setup_cluster(struct mddev *mddev, int nodes);
@@ -932,7 +932,7 @@ static inline void rdev_dec_pending(struct md_rdev *rdev, struct mddev *mddev)
 	}
 }
 
-extern struct md_cluster_operations *md_cluster_ops;
+extern const struct md_cluster_operations *md_cluster_ops;
 static inline int mddev_is_clustered(struct mddev *mddev)
 {
 	return mddev->cluster_info && mddev->bitmap_info.nodes > 1;