RFR: 8265064: Move clearing and setting of members into helpers in ReservedSpace [v2]

[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.

Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:

  Thomas review.
  
  Using initialize_members() in clear_members().

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3435/files
  - new: https://git.openjdk.java.net/jdk/pull/3435/files/de6e48b9..cd9f0816

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3435&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3435&range=00-01

  Stats: 6 lines in 1 file changed: 0 ins; 5 del; 1 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