RFR 8191416: (bf) DirectByteBuffer extends MappedByteBuffer, confuses instanceof tests

[Brian Burkhalter]

https://bugs.openjdk.java.net/browse/JDK-8191416
http://cr.openjdk.java.net/~bpb/8191416/webrev.00/

As suggested in the issue description, change force(), load(), and isLoaded() to be no-ops when there is no backing file instead of throwing UnsupportedOperationException.

Thanks,

Brian