RFR: 8231774: ZGC: ZVirtualMemoryManager unmaps incorrect address

[Kim Barrett]

> On Oct 2, 2019, at 5:53 PM, Per Liden <per.liden at oracle.com> wrote:
> 
> When failing to map the requested address, map() in ZVirtualMemoryManager.cpp, incorrectly calls unmap(start, size) instead of unmap(res, size).
> 
> Bug: https://bugs.openjdk.java.net/browse/JDK-8231774
> Webrev: http://cr.openjdk.java.net/~pliden/8231774/webrev.0
> 
> /Per

Looks good.