fix: return the read sectors length when it reads the expansion part

ref: longhorn/longhonr 6899

Signed-off-by: Jack Lin <[email protected]>

pkg/replica/diff_disk.go

@@ -237,7 +237,7 @@ func (d *diffDisk) read(target byte, buf []byte, startOffset int64, startSector
 
 	// May read the expanded part
 	if offset >= size {
-		return 0, nil
+		return int(bufLength), nil
 	}
 	var newBuf []byte
 	if bufLength > size-offset {