ZGC: Fix reserve high/low accounting

[Stefan Karlsson]

Looks good.

StefanK

On 2018-06-05 13:30, Per Liden wrote:
> We had incorrectly swapped the reserved high/low accounting.
> 
> http://cr.openjdk.java.net/~pliden/zgc/fix_reserve_high_low/webrev.0
> 
> /Per