RFR: 8235119: Incomplete initialization of scan_top values results in out-of-bounds scanning of regions

[Stefan Johansson]

Hi,

Please review this fix for JDK-8235119, it's intended for both JDK 14 
and 15.

Webrev: http://cr.openjdk.java.net/~sjohanss/8235119/00/
Issue: https://bugs.openjdk.java.net/browse/JDK-8235119

Summary
When we parallelized the evacuation preparation in JDK-8141637 we made a 
small change to how we initialize the scan state. The problem with this 
change was that free and unavailable regions didn't get cleared and 
reset as before. This will lead to regions being allocated during the GC 
can have an inconsistent state.

This fix reverts a small part of JDK-8141637, so that we continue to 
clear and reset all regions regardless of state and then just update the 
state for the regions needing it.

Testing
Functional testing in mach5 tier1-3 and Kim has done manual verification 
with tests that previously reproduced the failure.

Also big thanks to Kim, Sangheon and Thomas for all the work with 
reproducing and digging into the issue.

Cheers,
Stefan