RFR: 8327097: GenShen: Align PLAB sizes down rather than up [v8]

[Kelvin Nilsen]

> When adjusting LAB sizes, round down rather than rounding up.  Otherwise, we may violate the ShenandoahHumongousThreshold bound.

Kelvin Nilsen has updated the pull request incrementally with one additional commit since the last revision:

  Respond to reviewer feedback

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/401/files
  - new: https://git.openjdk.org/shenandoah/pull/401/files/77663512..1e28350f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=07
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=401&range=06-07

  Stats: 48 lines in 6 files changed: 7 ins; 16 del; 25 mod
  Patch: https://git.openjdk.org/shenandoah/pull/401.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/401/head:pull/401

PR: https://git.openjdk.org/shenandoah/pull/401