RFR: JDK-8283670: gtest os.release_multi_mappings_vm is still racy

[Thomas Stuefe]

[JDK-8280940](https://bugs.openjdk.java.net/browse/JDK-8280940) fixed up os.release_multi_mappings_vm to make its handling of multiple neighboring reservations less racy, but I forgot to fix up the reserve_multiple() function, which follows the same pattern and is still racy. It reserves an area, releases it, then re-reserves stripes in that area.

We see intermittent problems on Linux PPC. This makes sense because we have here 64k pages, and therefore the stripe size is large, and we have a higher chance of someone grabbing that address space concurrently between the release and the stripe-wise re-reserve.

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Commit messages:
 - JDK-8283670-os.release_multi_mappings_vm-is-still-racy

Changes: https://git.openjdk.java.net/jdk/pull/7953/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7953&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8283670
  Stats: 44 lines in 1 file changed: 20 ins; 8 del; 16 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7953.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7953/head:pull/7953

PR: https://git.openjdk.java.net/jdk/pull/7953