RFR: 8155082: Refactor mutator region restriction

[Stefan Johansson]

Hi,

Please review this small refactoring for:
https://bugs.openjdk.java.net/browse/JDK-8155082

Webrev:
http://cr.openjdk.java.net/~sjohanss/8155082/hotspot.00/

Summary:
The method is_young_list_full is only used to determine if we should 
continue to allocate mutator regions so it makes sense to rename it to 
make this more obvious.

Testing:
JPRT

Thanks,
Stefan