Fix disk usage block size

nilmdb/utils/diskusage.py

@@ -22,7 +22,7 @@ def du(path):
     files in the process of being removed."""
     try:
         st = os.stat(path)
-        size = st.st_blocks * st.st_blksize
+        size = st.st_blocks * 512
         if os.path.isdir(path):
             for thisfile in os.listdir(path):
                 filepath = os.path.join(path, thisfile)