RFR: 8366434: THP not working properly with G1 after JDK-8345655

[Stefan Johansson]

Please review this fix to enable transparent huge pages for G1,

**Summary**
In [JDK-8345655](https://bugs.openjdk.org/browse/JDK-8345655) we refactored the memory reservation code to be more maintainable and easier to follow. When doing this one of the code paths changed to always pass in `os::vm_page_size()` where it previously had used a page size provided by the caller.

Even if the alignment for `ReservedSpaces` created this way show that they should be aligned to support large pages the page size member for the `ReservedSpace` does not convey that we want large pages for the space. In G1 when using `G1PageBaseVirtualSpace` we use the above mentioned page size as the alignment for the reservation. This leads to reservations (made using the API) not being THP eligible even if `-XX:+UseTransparentHugePages` is specified.

This is only an issue when the system is configured with the THP mode `madvise`. If the mode is `always`, we will get THP eligible reservations. So a fairly simple workaround for this issue (given you have access to configuring your system) is to configure the THP mode to always.

The fix is to simply change back to the old behavior and pass in the user provided page size to the `ReservedSpace`. We've also added a test that verifies that we try to back the heap with transparent huge pages when `-XX:+UseTransparentHugePages` is specified on the command-line.

**Testing**
* Mach5 testing tier1-tier5
* Manual testing of the new test both locally and on mach5. Making sure it has been executed on system with both `madvise` and `always` configured. Also made sure the test actually failed without the fix.

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Commit messages:
 - Typo
 - StefanK review
 - Test parsing smaps in process
 - 8366434: THP not working properly with G1 after JDK-8345655

Changes: https://git.openjdk.org/jdk/pull/27051/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27051&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366434
  Stats: 156 lines in 3 files changed: 154 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/27051.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27051/head:pull/27051

PR: https://git.openjdk.org/jdk/pull/27051