RFR: 8299193: Micro-optimization in Buffer

[Per Minborg]

This PR proposed a micro-optimization in Buffer. This change will benefit several subclasses as well.

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Commit messages:
 - Declare Buffer.capaity final

Changes: https://git.openjdk.org/jdk/pull/11755/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11755&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8299193
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/11755.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11755/head:pull/11755

PR: https://git.openjdk.org/jdk/pull/11755