Correct inaccurate BlocksRemoved metric on DataNode side

case DatanodeProtocol.DNA_INVALIDATE:
      //
      // Some local block(s) are obsolete and can be 
      // safely garbage-collected.
      //
      Block toDelete[] = bcmd.getBlocks();
      try {
        // using global fsdataset
        dn.getFSDataset().invalidate(bcmd.getBlockPoolId(), toDelete);
      } catch(IOException e) {
        // Exceptions caught here are not expected to be disk-related.
        throw e;
      }
      dn.metrics.incrBlocksRemoved(toDelete.length);
      break;