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 <rgoldwyn@suse.com>
Cc: NeilBrown <neilb@suse.com>
Acked-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/bitmap.c

@@ -210,10 +210,6 @@ static int write_sb_page(struct bitmap *bitmap, struct page *page, int wait)
 	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;