Make block size from NNThroughputBenchmark configurable

When test NNThroughputBenchmark, get following error logs.

2020-10-26 20:51:25,781 ERROR namenode.NNThroughputBenchmark: StatsDaemon 43 failed:
org.apache.hadoop.ipc.RemoteException(java.io.IOException): Specified block size is less than configured minimum value (dfs.namenode.fs-limits.min-block-size): 16 < 1048576
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.startFileInt(FSNamesystem.java:2514)
at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.startFile(FSNamesystem.java:2452)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.createOriginal(NameNodeRpcServer.java:824)
at org.apache.hadoop.hdfs.server.namenode.ProtectionManager.create(ProtectionManager.java:344)
at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.create(NameNodeRpcServer.java:792)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.create(ClientNamenodeProtocolServerSideTranslatorPB.java:326)
at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:528)
at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:1073)
at org.apache.hadoop.ipc.Server$RpcCall.run(Server.java:1020)
at org.apache.hadoop.ipc.Server$RpcCall.run(Server.java:948)
at java.security.AccessController.doPrivileged(Native Method)
at javax.security.auth.Subject.doAs(Subject.java:422)
at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:2002)
at org.apache.hadoop.ipc.Server$Handler.run(Server.java:2985)

at org.apache.hadoop.ipc.Client.getRpcResponse(Client.java:1562)
at org.apache.hadoop.ipc.Client.call(Client.java:1508)
at org.apache.hadoop.ipc.Client.call(Client.java:1405)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Invoker.invoke(ProtobufRpcEngine.java:233)
at org.apache.hadoop.ipc.ProtobufRpcEngine$Invoker.invoke(ProtobufRpcEngine.java:118)
at com.sun.proxy.$Proxy9.create(Unknown Source)
at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolTranslatorPB.create(ClientNamenodeProtocolTranslatorPB.java:281)
at sun.reflect.GeneratedMethodAccessor10.invoke(Unknown Source)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.hadoop.io.retry.RetryInvocationHandler.invokeMethod(RetryInvocationHandler.java:422)
at org.apache.hadoop.io.retry.RetryInvocationHandler$Call.invokeMethod(RetryInvocationHandler.java:165)
at org.apache.hadoop.io.retry.RetryInvocationHandler$Call.invoke(RetryInvocationHandler.java:157)
at org.apache.hadoop.io.retry.RetryInvocationHandler$Call.invokeOnce(RetryInvocationHandler.java:95)
at org.apache.hadoop.io.retry.RetryInvocationHandler.invoke(RetryInvocationHandler.java:359)
at com.sun.proxy.$Proxy10.create(Unknown Source)
at org.apache.hadoop.hdfs.server.namenode.NNThroughputBenchmark$CreateFileStats.executeOp(NNThroughputBenchmark.java:597)
at org.apache.hadoop.hdfs.server.namenode.NNThroughputBenchmark$StatsDaemon.benchmarkOne(NNThroughputBenchmark.java:428)
at org.apache.hadoop.hdfs.server.namenode.NNThroughputBenchmark$StatsDaemon.run(NNThroughputBenchmark.java:412)

Because NN has start and serves, we should make block size of client benchmark configurable, and that will be convenient