Fix CheckpointRDD java.io.FileNotFoundException when calling getPreferredLocations

When RDD's checkpoint is enabled, calling RDD's `preferredLocations()` will using CheckpointRDD's `getPreferredLocations()`  to replace original RDD's. And in CheckpointRDD's  `getPreferredLocations()`, here `checkpointPath` is only RDD's directory, not the partition's file path, so calling these two function:

```scala
    val status = fs.getFileStatus(new Path(checkpointPath))
    val locations = fs.getFileBlockLocations(status, 0, status.getLen)
```

will get following exception:

    Exception in thread "DAGScheduler" java.io.FileNotFoundException: File does not exist: /user/root/checkpoint/00c278cf-10bd-4e68-a963-11a1de8f0129/rdd-50
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getBlockLocationsUpdateTimes(FSNamesystem.java:1312)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getBlockLocationsInt(FSNamesystem.java:1258)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getBlockLocations(FSNamesystem.java:1231)
    at org.apache.hadoop.hdfs.server.namenode.FSNamesystem.getBlockLocations(FSNamesystem.java:1213)
    at org.apache.hadoop.hdfs.server.namenode.NameNodeRpcServer.getBlockLocations(NameNodeRpcServer.java:392)
    at org.apache.hadoop.hdfs.protocolPB.ClientNamenodeProtocolServerSideTranslatorPB.getBlockLocations(ClientNamenodeProtocolServerSideTranslatorPB.java:170)
    at org.apache.hadoop.hdfs.protocol.proto.ClientNamenodeProtocolProtos$ClientNamenodeProtocol$2.callBlockingMethod(ClientNamenodeProtocolProtos.java:44064)
    at org.apache.hadoop.ipc.ProtobufRpcEngine$Server$ProtoBufRpcInvoker.call(ProtobufRpcEngine.java:453)
    at org.apache.hadoop.ipc.RPC$Server.call(RPC.java:1002)
    at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1695)
    at org.apache.hadoop.ipc.Server$Handler$1.run(Server.java:1691)
    at java.security.AccessController.doPrivileged(Native Method)
    at javax.security.auth.Subject.doAs(Subject.java:396)
    at org.apache.hadoop.security.UserGroupInformation.doAs(UserGroupInformation.java:1408)
    at org.apache.hadoop.ipc.Server$Handler.run(Server.java:1689)

    at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
    at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
    at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
    at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
    at org.apache.hadoop.ipc.RemoteException.instantiateException(RemoteException.java:90)
    at org.apache.hadoop.ipc.RemoteException.unwrapRemoteException(RemoteException.java:57)
    at org.apache.hadoop.hdfs.DFSClient.callGetBlockLocations(DFSClient.java:935)
    at org.apache.hadoop.hdfs.DFSClient.getLocatedBlocks(DFSClient.java:923)
    at org.apache.hadoop.hdfs.DFSClient.getBlockLocations(DFSClient.java:972)
    at org.apache.hadoop.hdfs.DistributedFileSystem.getFileBlockLocations(DistributedFileSystem.java:191)
    at org.apache.hadoop.hdfs.DistributedFileSystem.getFileBlockLocations(DistributedFileSystem.java:184)
    at spark.rdd.CheckpointRDD.getPreferredLocations(CheckpointRDD.scala:47)
    ....

So here RDD's directory path should change to partition's file path