RFR: JDK-8266419: [aix] in mmap mode, os::attempt_reserve_memory_at() fails to handle wrong mapping address

[Thomas Stuefe]

On AIX, in mmap() mode, if the mmap call succeeds but not at the requested address, the mapping should be unmapped and the operation should count as error - like it is done on all other platforms.

(AFAICS this is a day zero bug in our port - mmap mode is seldomly used).

Tested: Nightlies at SAP with and without 4K pages.

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Commit messages:
 - start

Changes: https://git.openjdk.java.net/jdk/pull/3834/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3834&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266419
  Stats: 15 lines in 1 file changed: 5 ins; 8 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3834.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3834/head:pull/3834

PR: https://git.openjdk.java.net/jdk/pull/3834