restrict deserialized classes in LeveldbConfigurationStore log decode

[naruto-lgtm]

Description of PR

LeveldbConfigurationStore.deserLogMutations reads the scheduler configuration mutation log back from the LevelDB store with a raw ObjectInputStream.readObject(). Anyone who can write the store directory (yarn.scheduler.configuration.leveldb-store.path) can replace the serialized LinkedList<LogMutation> with a gadget payload, and the RM will instantiate arbitrary Serializable classes off the classpath on the next load/recovery.

The sibling ZKConfigurationStore already decodes the same LinkedList<LogMutation> through commons-io ValidatingObjectInputStream with an explicit class allowlist; the LevelDB store was left on the unrestricted path. This change applies the same allowlist (LinkedList, LogMutation, HashMap, String) inside the decode helper so the restriction lives next to the read rather than relying on the store being trusted.

How was this patch tested?

Round-tripped a real LinkedList<LogMutation> (a HashMap of updates plus a user string) through the patched helper - it deserializes unchanged. A serialized object of a class outside the allowlist is now rejected with InvalidClassException ("Class name not accepted") instead of being constructed. Built hadoop-yarn-server-resourcemanager with the change.

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