RFR (S): 8077255: TracePageSizes output reports wrong page size on Windows with G1

[Thomas Schatzl]

Hi all,

  can I have reviews for the following change that fixes the output of
-XX:+TracePageSizes to report the expected values for page size.
Instead of returning the value of the page size to be used for
allocations, this returned the OS'es allocation granularity which is the
same on all but Windows, where this test subsequently fails.

I am not completely sure why this did not fail with the corresponding
push. The test most of the time still passes.

The change simply calculates the preferred page size and passes it to
the ReservedSpace constructor where it is used if set. Otherwise, the
ReservedSpace constructor calculates that value by itself as before.

CR:
https://bugs.openjdk.java.net/browse/JDK-8077255
Webrev:
http://cr.openjdk.java.net/~tschatzl/8077255/webrev.0/
Testing:
JPRT (supposedly running that test), manual verification of the -XX:
+TracePageSizes output before and after the change with and without -XX:
+UseLargePages.

Thanks,
  Thomas