RFR: 8367317: ZGC: ZVirtualMemoryReserver::force_reserve_discontiguous arithmetic underflow [v2]

[Axel Boldt-Christmas]

> ZVirtualMemoryReserver::force_reserve_discontiguous is a debug only API used to simulate and test a discontinuous heap. The loop variable `end` is incorrectly checked which may lead to an underflow.
> 
> In our normal testing we never observe this as the address space is always available. We observed this issue when running a hardened build (like ASAN) which had already mmapped the address space we tried to reserve.
> 
> Rather than always jump two reserve sizes in its search, it searches every reserve size, moving an extra reserve size every time it succeeds with a reservation (so that there always is a gap).  Searching in reserve sized steps is not really a requirement here, we could have just used a granule sized step.

Axel Boldt-Christmas has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains two additional commits since the last revision:

 - Merge tag 'jdk-26+17' into JDK-8367317
   
   Added tag jdk-26+17 for changeset 2aafda19
 - 8367317: ZGC: ZVirtualMemoryReserver::force_reserve_discontiguous arithmetic underflow

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/27207/files
  - new: https://git.openjdk.org/jdk/pull/27207/files/825941e0..6c77150f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27207&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27207&range=00-01

  Stats: 162142 lines in 1784 files changed: 132737 ins; 18333 del; 11072 mod
  Patch: https://git.openjdk.org/jdk/pull/27207.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27207/head:pull/27207

PR: https://git.openjdk.org/jdk/pull/27207