RFR(T) 8219244: NMT: Move safepoint related memory allocations from mtInternal to mtSafepoint

[zgu at redhat.com]

There is only one instance I can find.

Bug: https://bugs.openjdk.java.net/browse/JDK-8219244
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8219244/webrev.00/

Test: 

  hotspot_nmt on Linux x64 (fastdebug and release)
  JDK-submit tests.

Thanks,

-Zhengyu