RFR: 8231774: ZGC: ZVirtualMemoryManager unmaps incorrect address

[Per Liden]

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