Erasure coding: properly handle start offset for internal blocks in a block group

  void actualGetFromOneDataNode(final DNAddrPair datanode,
    ...
      LocatedBlock block = getBlockAt(blockStartOffset);
    ...
      fetchBlockAt(block.getStartOffset());

The blockStartOffset here is from inner block. For parity blocks, the offset will overlap with the next block group, and we may end up with fetching wrong block. So we have to assign a meaningful start offset for internal blocks in a block group, especially for parity blocks.