RFR: 8313743: Make fields final in sun.nio.ch

[Andrey Turbanov]

Found a few fields in `sun.nio.ch` package which could be declared `final`.

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Commit messages:
 - [PATCH] Make fields final in sun.nio.ch
 - [PATCH] Make fields final in sun.nio.ch

Changes: https://git.openjdk.org/jdk/pull/14980/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14980&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313743
  Stats: 39 lines in 9 files changed: 0 ins; 18 del; 21 mod
  Patch: https://git.openjdk.org/jdk/pull/14980.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14980/head:pull/14980

PR: https://git.openjdk.org/jdk/pull/14980