RFR: JDK-8283670: gtest os.release_multi_mappings_vm is still racy [v2]

[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.

Thomas Stuefe has updated the pull request incrementally with one additional commit since the last revision:

  Davids feedback

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7953/files
  - new: https://git.openjdk.java.net/jdk/pull/7953/files/669cb5d3..6846a4e9

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7953&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7953&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 2 del; 1 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