RFR: NMT: Don't record uncommits unconditionally

[Stefan Karlsson]

Hi all,

Please review this patch to fix a bug in the recent NMT implementation 
for ZGC.

http://cr.openjdk.java.net/~stefank/zgc/zNMTBugFix/webrev.01

The uncommit path was executed even when NMT was turned off. Because of 
this we hit the following assertion when pages were detached:

#  Internal Error (.../services/memTracker.hpp:231), pid=12751, tid=2738
#  assert(tracking_level() >= NMT_summary) failed: Check by caller

Thanks,
StefanK