RFR(XS) 8194934: NMT: Remove MemTracker::get_virtual_memory_xxx_tracker(), create Tracker in place

[Zhengyu Gu]

MemTracker is a stack object, it locks ThreadCrtical in its constructor, 
and unlock it in destructor.

Using MemTracker::get_virtual_memory_XXX_tracker() to acquire a tracker, 
causes an additional lock/unlock of ThreadCritical.

This patch creates MemTracker in place to avoid the extra lock/unlock.

Bug: https://bugs.openjdk.java.net/browse/JDK-8194934
Webrev: http://cr.openjdk.java.net/~zgu/8194934/webrev.00/

Test:

   hotspot_tier1_runtime on Linux 64 (fastdebug and release)

   Verified minimal VM build.


Thanks,

-Zhengyu