[8u] GenericTaskQueue destructor is incorrect

This is on behalf of <wattsun@tencent.com>.

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When do some g1 gc improvement in jdk8u, we found a potential bug in class GenericTaskQueue.

Memory allocated by _array_allocator [1] in class GenericTaskQueue will be freed when ~GenericTaskQueue [2] is called.

And will be freed again when ~ArrayAllocator [3] called again due to free_in_destructor = true.
 
[1] http://hg.openjdk.java.net/jdk8u/jdk8u-dev/hotspot/file/002b9c947f0f/src/share/vm/utilities/taskqueue.hpp#l259
[2] http://hg.openjdk.java.net/jdk8u/jdk8u-dev/hotspot/file/002b9c947f0f/src/share/vm/utilities/taskqueue.hpp#l432
[3] http://hg.openjdk.java.net/jdk8u/jdk8u-dev/hotspot/file/002b9c947f0f/src/share/vm/memory/allocation.hpp#l736
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