RFR: 8196217: NMT: add_committed_regions doesn't merge succeeding regions

[Stefan Karlsson]

Hi all,

Please review this patch to fix merging of committed regions in NMT. 
Before this patch, only the regions without 'next' regions were merged.

http://cr.openjdk.java.net/~stefank/8196217/webrev.01/

This was found while running stress tests on ZGC:
http://mail.openjdk.java.net/pipermail/zgc-dev/2018-January/000127.html

Tested with the new test in the patch and the rest of the tests in 
test/hotspot/jtreg/runtime/NMT.

Thanks,
StefanK