Do not trust bad block reports from clients

reportBadBlocks() is implemented by both ClientNamenodeProtocol and DatanodeProtocol. When DFSClient is calling it, a faulty client can cause data availability issues in a cluster.

In the past we had such an incident where a node with a faulty NIC was randomly corrupting data. All clients ran on the machine reported all accessed blocks and all associated replicas to be corrupt. More recently, a single faulty client process caused a small number of missing blocks. In all cases, actual data was fine.

The bad block reports from clients shouldn't be trusted blindly. Instead, the namenode should send a datanode command to verify the claim. A bonus would be to keep the record for a while and ignore repeated reports from the same nodes.

At minimum, there should be an option to ignore bad block reports from clients, perhaps after logging it. A very crude way would be to make it short out in ClientNamenodeProtocolServerSideTranslatorPB#reportBadBlocks(). More sophisticated way would be to check for the datanode user name in FSNamesystem#reportBadBlocks() so that it can be easily logged, or optionally do further processing.