RFR: 8278756: Parallel: Drop PSOldGen::_reserved

[Albert Mingkun Yang]

Simple change of removing `PSOldGen::_reserved` and constructing reserved mem region on the fly.

Test: hotspot_gc, tier1-3

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Commit messages:
 - ps-reserve

Changes: https://git.openjdk.java.net/jdk/pull/6829/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6829&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278756
  Stats: 68 lines in 9 files changed: 16 ins; 30 del; 22 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6829.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6829/head:pull/6829

PR: https://git.openjdk.java.net/jdk/pull/6829