[8u40] RFR (backport) 8059100: SIGSEGV VirtualMemoryTracker::remove_released_region

[Coleen Phillimore]

Summary: Disallow increasing native memory tracking level
Reviewed-by: hseigel, ctornqvi, gtriantafill

Fix has been tested in jdk9 repository for a week (two days a week have 
NMT on) and patch applied cleanly.

open webrev at http://cr.openjdk.java.net/~coleenp/8059100_8u40/
bug link https://bugs.openjdk.java.net/browse/JDK-8059100

Thanks,
Coleen