RFR: JDK-8313319: [linux] mmap should use MAP_FIXED_NOREPLACE if available

[Thomas Stuefe]

os::attempt_reserve_memory_at() is used to map memory at a specific address that we fancy. It is not used to replace an existing mapping (hence we omit MAP_FIXED). 

But it may result in the kernel creating a mapping at a different address if our wish address is blocked; we then have to unmap again. That mmap-munmap cycle is unnecessary.

MAP_FIXED_NOREPLACE exists since Linux 4.17 and prevents that unnecessary cycle . We should use that if available.

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Commit messages:
 - DK-8313319-MAP_FIXED_NOREPLACE

Changes: https://git.openjdk.org/jdk/pull/15170/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15170&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8313319
  Stats: 49 lines in 2 files changed: 47 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/15170.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15170/head:pull/15170

PR: https://git.openjdk.org/jdk/pull/15170