JDK 9 RFR of test-only 8180085: (ch) java/nio/channels/SocketChannel/VectorIO.java: add debug instrumentation

[Brian Burkhalter]

Please review at your convenience.

Issue:	https://bugs.openjdk.java.net/browse/JDK-8180085
Patch:	http://cr.openjdk.java.net/~bpb/8180085/webrev.00/

Add some print statements potentially useful in solving https://bugs.openjdk.java.net/browse/JDK-8177801.

Thanks,

Brian