RFR: 8315765: G1: Incorrect use of G1LastPLABAverageOccupancy

[Albert Mingkun Yang]

Simple fix in calculating the number of plab refills. The default value is 50, so no real effect.

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Commit messages:
 - g1-plab-occupancy

Changes: https://git.openjdk.org/jdk/pull/15587/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15587&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315765
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/15587.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15587/head:pull/15587

PR: https://git.openjdk.org/jdk/pull/15587