RFR: 8040793: vmTestbase/nsk/monitoring/stress/lowmem fails on calling isCollectionUsageThresholdExceeded()

[Albert Mingkun Yang]

Fix a simple bug in Serial eden-resize logic by clamping it by its max value.

Test: observed two failures with 100 iterations of `vmTestbase/nsk/monitoring/stress/lowmem` before and no failures for 500 iterations afterwards.

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Commit messages:
 - fix

Changes: https://git.openjdk.org/jdk/pull/12516/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12516&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8040793
  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12516.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12516/head:pull/12516

PR: https://git.openjdk.org/jdk/pull/12516