RFR: 8252514: ZGC: Physical memory splitting is broken

[Stefan Karlsson]

Hi all,

Please review this patch to fix physical memory splitting in ZGC.

https://cr.openjdk.java.net/~stefank/8252514/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8252514

JDK-8247759 changed the implementation to use a GrowableArray. One part 
of that patch cached the value of _segments.at(index) inside 
commit_segment and uncommit_segment. This is incorrect, since resizing 
the array causes the backing memory to be deallocated, causing the the 
local copy to point to garbage.

I've reverted that change, and now always call _segments.at(index) instead.

This was found while experimenting with physical memory allocations that 
were likely to fail. I've verified that the failures stops reproducing 
with this patch.

Thanks,
StefanK