RFR: 8260262: Use common code in function unmap_shared() in perfMemory_posix.cpp

[Harold Seigel]

Please review this change to use common code in function unmap_shared() in perfMemory_posix.cpp, to fix JDK-8260262.  The change calls munmap() directly to deallocate the memory because functions mmap_create_shared() and mmap_attach_shared() call mmap() directly to allocate the memory.

The change was tested by running Mach5 tiers 1-2 on Linux, MacOS, and Windows, and Mach5 tiers 3-5 on Linux x64 and MacOS x64.

Note that testing on AIX is needed.

Thanks, Harold

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Commit messages:
 - 8260262: Use common code in function unmap_shared() in perfMemory_posix.cpp

Changes: https://git.openjdk.java.net/jdk/pull/4995/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4995&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260262
  Stats: 17 lines in 1 file changed: 8 ins; 3 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4995.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4995/head:pull/4995

PR: https://git.openjdk.java.net/jdk/pull/4995