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

[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.

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Commit messages:
 - 8261230-test-fix
 - 8261230: GC tracing of page sizes are wrong in a few places

Changes: https://git.openjdk.java.net/jdk/pull/2486/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2486&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261230
  Stats: 33 lines in 4 files changed: 26 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