BlockManager.metaSave does not distinguish between "under replicated" and "missing" blocks

Currently, BlockManager.metaSave method (which is called by "-metasave" dfs CLI command) reports both "under replicated" and "missing" blocks under same metric Metasave: Blocks waiting for reconstruction: as shown on below code snippet:

   synchronized (neededReconstruction) {
      out.println("Metasave: Blocks waiting for reconstruction: "
          + neededReconstruction.size());
      for (Block block : neededReconstruction) {
        dumpBlockMeta(block, out);
      }
    }

neededReconstruction is an instance of LowRedundancyBlocks, which actually wraps 5 priority queues currently. 4 of these queues store different under replicated scenarios, but the 5th one is dedicated for corrupt/missing blocks.

Thus, metasave report may suggest some corrupt blocks are just under replicated. This can be misleading for admins and operators trying to track block missing/corruption issues, and/or other issues related to BlockManager metrics.

I would like to propose a patch with trivial changes that would report corrupt blocks separately.