Let client retry the same NN when "dfs.client.test.drop.namenode.response.number" is enabled

Currently when dfs.client.test.drop.namenode.response.number is enabled for testing, the client will start failover and try the other NN. But in most of the testing cases we do not need to trigger the client failover here since if the drop-response number is >1 the next response received from the other NN will also be dropped. We can let the client just simply retry the same NN.