DDF: fix writing metadata updates.

Recent commit 273989b93a
skipped writing some large blocks of 0xFF, but didn't seek
over the space, so subsequent data was written wrongly.

When we don't write, we need to seek.

Signed-off-by: NeilBrown <neilb@suse.de>

super-ddf.c

@@ -2907,7 +2907,8 @@ static int __write_ddf_structure(struct dl *d, struct ddf_super *ddf, __u8 type,
 			}
 			if (write(fd, null_aligned, togo) < 0)
 				break;
-		}
+		} else
+			lseek(fd, conf_size, SEEK_CUR);
 	}
 	if (i <= n_config)
 		goto out;