HDFS-15660. StorageTypeProto is not compatiable between 3.x and 2.6. Contributed by Ryan Wu.

hadoop-hdfs-project/hadoop-hdfs-client/src/main/proto/hdfs.proto

@@ -198,7 +198,7 @@ message StorageTypeQuotaInfosProto {
 }
 
 message StorageTypeQuotaInfoProto {
-  required StorageTypeProto type = 1;
+  optional StorageTypeProto type = 1 [default = DISK];
   required uint64 quota = 2;
   required uint64 consumed = 3;
 }