fix `Address::sub`

src/block.rs

@@ -177,7 +177,7 @@ impl<S: Size> Sub for Address<S> {
     type Output = Address<S>;
     fn sub(self, rhs: Address<S>) -> Address<S> {
         Address::new(
-            self.block + rhs.block,
+            self.block - rhs.block,
             self.offset as isize - rhs.offset as isize,
         )
     }

src/fs.rs

@@ -229,6 +229,11 @@ mod tests {
     #[test]
     fn file_len() {
         let file = RefCell::new(File::open("ext2.img").unwrap());
+        assert_eq!(
+            Address::<Size512>::from(2048_usize)
+                - Address::<Size512>::from(1024_usize),
+            Address::<Size512>::new(2, 0)
+        );
         assert_eq!(
             unsafe {
                 file.slice_unchecked(