RFR: Avoid write barrier during full GC

[Zhengyu Gu]

This one line change to avoid write barrier during full GC.

Webrev: 
http://cr.openjdk.java.net/~zgu/shenandoah/avoid_wb_full_gc/weberv.00/


Test:
   tier3_gc_shenandoah (fastdebug and release)

Thanks,

-Zhengyu