RFR(S): 8251860: ClassLoaderData::loaded_classes_do fails with "assert(ZAddress::is_marked(addr)) failed: Should be marked"

[calvin.cheung at oracle.com]

bug: https://bugs.openjdk.java.net/browse/JDK-8251860

webrev: http://cr.openjdk.java.net/~ccheung/jdk16/8251860/webrev.00/

Please refer to the bug report for an analysis of the crash.

The proposed fix is to acquire the ClassLoaderDataGraph_lock before 
calling ClassLoaderDataGraph::loaded_classes_do(&link_closure). The 
link_closure will not do the actual linking but just to store the 
InstanceKlass'es in an array and increment the keep_alive counter of the 
class_loader_data of each InstanceKlass. After the call to 
ClassLoaderDataGraph::loaded_classes_do, each InstanceKlass in the array 
will be linked and the keep_alive counter of the class_loader_data will 
be decremented.

Testing:
     - tested about 40 times on the MacOS host where the crash was seen
        (before the fix, the crash was seen about once in 20 runs)
     - Tier1 - 4.
thanks,
Calvin