[jdk16] RFR: 8259032: MappedMemorySegmentImpl#makeMappedSegment() ignores Unmapper#pagePosition

[Maurizio Cimadamore]

Hi,
this patch fixes an oversight where the address() method in FileChannelImpl.Unmapper does not take into account the value of `pagePosition` - this leads to a situation in which, effectively, the specified offset when mapping the segment is ignored by the runtime (because base address will always be aligned to some known quantity - which is OS/platform dependent).

To test this I had to break open into FileChannelImpl and ready the granularity.

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Commit messages:
 - Fix issue in Unmapper implementation not including `pagePosition` in address computation

Changes: https://git.openjdk.java.net/jdk16/pull/84/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk16&pr=84&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8259032
  Stats: 32 lines in 2 files changed: 30 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk16/pull/84.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk16 pull/84/head:pull/84

PR: https://git.openjdk.java.net/jdk16/pull/84