RFR: 8309882: LinkedHashMap adds an errant serializable field

[Stuart Marks]

Make `putMode` be a transient field.

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Commit messages:
 - 8309882: LinkedHashMap adds an errant serializable field

Changes: https://git.openjdk.org/jdk/pull/14433/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14433&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309882
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/14433.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14433/head:pull/14433

PR: https://git.openjdk.org/jdk/pull/14433