RFR: 8261230: GC tracing of page sizes are wrong in a few places [v2]

[Stefan Johansson]

> The usage of `os::trace_page_sizes()` and friends are wrongly assuming that we always get the page size requested and needs to be updated. This is done by using the helper `ReservedSpace::actual_reserved_page_size()` instead of blindly trusting we get what we ask for. I have plans for the future to get rid of this helper and instead record the page size used in the `ReservedSpace`, but for now the helper is good enough. 
> 
> In G1 we used the helper but switched the order of the page size and the alignment parameter, which in turn helped the test to pass since the alignment will match the page size we expect in the test. The test had to be improved to recognize mapping failures.

Stefan Johansson has updated the pull request incrementally with one additional commit since the last revision:

  Albert review
  
  Renamed helper to improve how the code read. Also extracted the failure check into a separate function.

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2486/files
  - new: https://git.openjdk.java.net/jdk/pull/2486/files/76faa2e4..b46f6a75

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2486&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2486&range=00-01

  Stats: 22 lines in 1 file changed: 15 ins; 1 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2486.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2486/head:pull/2486

PR: https://git.openjdk.java.net/jdk/pull/2486