RFR: Force termination logic is inverted

[Roman Kennke]

Duh. So our conc-mark-degen has been broken all along?! wtf..

I think the patch looks sane. Let Aleksey please ack it too.

Roman

> When should_force_termination() returns true, termination should be
> terminated even there are remaining tasks. Current logic does the
> opposite, it jumps right back for stealing works, e.g. returning false.
> Which means, it actually drains task queues, instead of leaving tasks
> there for degen cycle.
> 
> Weberev:
> http://cr.openjdk.java.net/~zgu/shenandoah/force_termination/webrev.00/
> 
> Test:
> 
>   tier3_gc_shenandoah (fastdebug and release)
> 
> Thanks,
> 
> -Zhengyu