RFR (T): 8224042 Add private alignDown method to MappedByteBuffer

[Andrew Dinn]

Please review this trivial change to MapperByteBuffer which encapsulates
the page align down operation in a suitably named method.

JIRA:   https://bugs.openjdk.java.net/browse/JDK-8224042
webrev: http://cr.openjdk.java.net/~adinn/8224042/webrev.00/

Testing:
This was successfully exercised by running

  jdk/java/nio/channels/FileChannel/MapTest.java

The test rounds down a sequence of sub-page offsets to successive pages
within a mapped buffer during testing of MappedByteBuffer method force().

regards,


Andrew Dinn
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Senior Principal Software Engineer
Red Hat UK Ltd
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