Summary
-------

Revert an erroneous change to the serialized format of `LinkedHashMap`.

Problem
-------

A new field `putMode` had been added to `LinkedHashMap` in the initial Sequenced Collection implementation (JDK-8266571). Since the class is serializable, this resulted in an unintended change to the serial format. 

Solution
--------

The field should be marked `transient` to restore the previous serial format.

Specification
-------------

    @@ -330,7 +330,7 @@ public class LinkedHashMap<K,V>
         static final int PUT_NORM = 0;
         static final int PUT_FIRST = 1;
         static final int PUT_LAST = 2;
    -    int putMode = PUT_NORM;
    +    transient int putMode = PUT_NORM;
     
         // Called after update, but not after insertion
         void afterNodeAccess(Node<K,V> e) {

(This has the effect of removing the `putMode` field from the serialized form of `LinkedHashMap` as documented on the serialized-form.html page of the specification.)