RFR: JDK-8271252: os::reserve_memory should not use mtOther as default NMT flag

[Thomas Stuefe]

Small cleanup.

We use mtOther as default NMT flag for os::reserve_memory():

`static char* reserve_memory(size_t bytes, bool executable = false, MEMFLAGS flags = mtOther);`

mtOther marks allocations coming from outside the VM. It is not a good default flag. Note that in the end mtOther then was ignored in os::reserve_memory() and it defaulted to mtNone anyway.

This trivial patch straightens the logic and uses `mtNone` as default. Note that since mtOther was ignored before, there is no behavioral change.

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Commit messages:
 - JDK-8271252-os-reserve-memory-should-not-default-to-mtOther

Changes: https://git.openjdk.org/jdk/pull/9287/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=9287&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8271252
  Stats: 6 lines in 2 files changed: 0 ins; 4 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/9287.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/9287/head:pull/9287

PR: https://git.openjdk.org/jdk/pull/9287