RFR (S) 8206471: Race with ConcurrentHashTable deleting items on insert with cleanup thread

[coleen.phillimore at oracle.com]

Summary: Only fetch Node::next once and use that result.

A racing thread could NULL next->next()->next().  The Node itself is 
stable until the write_synchronize() but the pointers may be updated.  
See bug for more detail.

open webrev at http://cr.openjdk.java.net/~coleenp/8206471.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8206471

Tested with SymbolTable changes and tests that failed.  Also tested with 
mach5 hs-tier1-5 (in progress).

This is actually Robbin's fix, and my review is that it looks good.

Thanks,
Coleen