RFR: 8265064: Move clearing and setting of members into helpers in ReservedSpace

[Stefan Johansson]

Currently there are a few places in `ReservedSpace` that updates a set of members directly during initialization. This could be refactored into helper functions to improve readability and also to pave way for some further cleanups.

This is one of a few cleanups I plan to do to enable a clean implementation of:
[JDK-8261527: Record page size used for underlying mapping in ReservedSpace](https://bugs.openjdk.java.net/browse/JDK-8261527)

**Testing**
Mach5 tier1 and tier2.

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Commit messages:
 - Self review.
 - 8265064: Move clearing and setting of members into helpers in ReservedSpace

Changes: https://git.openjdk.java.net/jdk/pull/3435/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3435&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265064
  Stats: 83 lines in 2 files changed: 43 ins; 27 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3435.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3435/head:pull/3435

PR: https://git.openjdk.java.net/jdk/pull/3435