8234805: (dc) Remove JNI upcall from DatagramChannel.receive implementation

[Chris Hegarty]

> ...
> 
>> http://cr.openjdk.java.net/~alanb/8234805/webrev/


I’ve reviewed and taken the changes for a test drive, and they seem
good.

Just a question. Is the use of ArraysSupport::vectorizedMismatch here
guaranteed to never return a tail (of elements to be checked)? If a
value other than -1 (no tail) is returned, it will not be seen by the
current code. If there is a guarantee of no tail (given the usage), then
maybe capture the value returned by vectorizedMismatch, and assert that
it is -1?

-Chris.