Replicated metadata only

Using replicated metadata for lost data recovery.

If you have only replicated metadata, you should be able to recover some, but not all, of the user data. Recover any data to be kept using normal file operations or erase the file. If you read a file in block-size chunks and get a failure return code and an EIO errno, that block of the file has been lost. The rest of the file may have useful data to recover, or it can be erased.