dm persistent data: switch exit_ro_spine to return void

In commit 4c7da06f ("dm persistent data: eliminate unnecessary
return values"), r value in exit_ro_spine will not change, so
exit_ro_spine doesn't need a return value.
Signed-off-by: Zhiqiang Liu <liuzhiqiang26@huawei.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/persistent-data/dm-btree-internal.h

@@ -68,7 +68,7 @@ struct ro_spine {
 };
 
 void init_ro_spine(struct ro_spine *s, struct dm_btree_info *info);
-int exit_ro_spine(struct ro_spine *s);
+void exit_ro_spine(struct ro_spine *s);
 int ro_step(struct ro_spine *s, dm_block_t new_child);
 void ro_pop(struct ro_spine *s);
 struct btree_node *ro_node(struct ro_spine *s);

drivers/md/persistent-data/dm-btree-spine.c

@@ -132,15 +132,13 @@ void init_ro_spine(struct ro_spine *s, struct dm_btree_info *info)
 	s->nodes[1] = NULL;
 }
 
-int exit_ro_spine(struct ro_spine *s)
+void exit_ro_spine(struct ro_spine *s)
 {
-	int r = 0, i;
+	int i;
 
 	for (i = 0; i < s->count; i++) {
 		unlock_block(s->info, s->nodes[i]);
 	}
-
-	return r;
 }
 
 int ro_step(struct ro_spine *s, dm_block_t new_child)