completeFile loops forever if the block's only replica has become corrupt

If a writer is appending to a block with replication factor 1, and that block has become corrupt, a reader will report the corruption to the NN. Then when the writer tries to complete the file, it will loop forever with an error like:

[junit] 2010-03-21 17:40:08,093 INFO namenode.FSNamesystem (FSNamesystem.java:checkFileProgress(1613)) - BLOCK* NameSystem.checkFileProgress: block blk_-4256412191814117589_1001

{blockUCState=COMMITTED, primaryNodeIndex=-1, replicas=[ReplicaUnderConstruction[127.0.0.1:56782|RBW]]}

has not reached minimal replication 1
[junit] 2010-03-21 17:40:08,495 INFO hdfs.DFSClient (DFSOutputStream.java:completeFile(1435)) - Could not complete file /TestReadWhileWriting/file1 retrying...

Should add tests that cover the case of a writer appending to a block that is corrupt while a reader accesses it.