RFR: 8354358: ZGC: ZPartition::prime handle discontiguous reservations correctly [v2]

[Axel Boldt-Christmas]

> Prior to [JDK-8350441](https://bugs.openjdk.org/browse/JDK-8350441) the VM would not have started if we received a discontiguous heap reservation with all reservations smaller than the inital heap capacity. Now we crash because `ZPartition::prime` does not take this into account.
> 
> However in contrast to the page cache, the mapped cache makes it trivial to support this scenario. So I propose fixing `ZPartition::prime` to handle any discontiguous heap reservation.
> 
> Can be provoked in a debug build by using ZForceDiscontiguousHeapReservations > 16
> `java -XX:+UseZGC -XX:ZForceDiscontiguousHeapReservations=17 -Xmx128m -Xms128m --version`
> 
> Currently running this through testing.

Axel Boldt-Christmas has updated the pull request incrementally with one additional commit since the last revision:

  Update Comment

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24589/files
  - new: https://git.openjdk.org/jdk/pull/24589/files/0abce51a..70b0e923

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24589&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24589&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24589.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24589/head:pull/24589

PR: https://git.openjdk.org/jdk/pull/24589