md-cluster: delete useless code

page->index already considers node offset. The node_offset calculation
in write_sb_page is useless and confusion.

Cc: Goldwyn Rodrigues <[e-mail]>
Cc: NeilBrown <[e-mail]>
Acked-by: Guoqing Jiang <[e-mail]>
Signed-off-by: Shaohua Li <[e-mail]>

drivers/md/bitmap.c

···
 	struct block_device *bdev;
 	struct mddev *mddev = bitmap->mddev;
 	struct bitmap_storage *store = &bitmap->storage;
-	int node_offset = 0;
-
-	if (mddev_is_clustered(bitmap->mddev))
-		node_offset = bitmap->cluster_slot * store->file_pages;
 
 	while ((rdev = next_active_rdev(rdev, mddev)) != NULL) {
 		int size = PAGE_SIZE;