FindBugs warning in branch-2: GlobalStorageStatistics defines non-transient non-serializable instance field map

Found in HDFS-14585

https://builds.apache.org/job/PreCommit-HDFS-Build/27024/artifact/out/branch-findbugs-hadoop-common-project_hadoop-common-warnings.html#Warnings_BAD_PRACTICE

Class org.apache.hadoop.fs.GlobalStorageStatistics defines non-transient non-serializable instance field map
Bug type SE_BAD_FIELD (click for details)
In class org.apache.hadoop.fs.GlobalStorageStatistics
Field org.apache.hadoop.fs.GlobalStorageStatistics.map
Actual type org.apache.hadoop.fs.StorageStatistics
In GlobalStorageStatistics.java

SE_BAD_FIELD: Non-transient non-serializable instance field in serializable class
This Serializable class defines a non-primitive instance field which is neither transient, Serializable, or java.lang.Object, and does not appear to implement the Externalizable interface or the readObject() and writeObject() methods. Objects of this class will not be deserialized correctly if a non-Serializable object is stored in this field.

Looking in my inbox, this warning has been there since Feburary 9, 2019