RFR: 8287905: Reduce runtime of java.nio microbenchmarks

[Claes Redestad]

Reduce runtime by reducing number of forks, iterations..

Most benchmarks in the java.nio packages already had sensible configuration, so this reduces estimated runtime by a small amount (~5h42m -> ~5s03m)

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Commit messages:
 - Reduce runtime java.nio

Changes: https://git.openjdk.org/jdk/pull/9183/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9183&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8287905
  Stats: 12 lines in 2 files changed: 12 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/9183.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9183/head:pull/9183

PR: https://git.openjdk.org/jdk/pull/9183