RFR: 8243486: ZGC: Adjust "Allocated" statistics to take undone page allocations into account

[Per Liden]

The "Allocated" statistics logged under gc+heap today includes page 
allocations that were later undone. Undoing page allocations is a fairly 
rare event, but this can still be a bit confusing to see. I propose that 
we adjust "Allocated" to take undone allocations into account.

Bug: https://bugs.openjdk.java.net/browse/JDK-8243486
Webrev: http://cr.openjdk.java.net/~pliden/8243486/webrev.0

/Per