RFR: JDK-8057057: Cleanup DIO JNI code

[Jen Dority]

issue: https://bugs.openjdk.java.net/browse/JDK-8057057
webrev: http://cr.openjdk.java.net/~jld/8057057/

Hi All,

Need a review for a bunch of JNI fixes. The code was missing 
Exception/NULL checks in a bunch of key places and I even found a 
missing return value.

Thanks,
Jen