[11u] RFR (S) 8245880: Shenandoah: check class unloading flag early in concurrent code root scan

[Aleksey Shipilev]

Original bug:
   https://bugs.openjdk.java.net/browse/JDK-8245880
   https://hg.openjdk.java.net/jdk/jdk/rev/3a8634555a01

The patch does apply cleanly to 11u, because the usual MutexLockerEx -> MutexLocker change. 
Reapplied the hunk by hand.

11u webrev:
   https://cr.openjdk.java.net/~shade/8245880/webrev.11u.01/

Testing: hotspot_gc_shenandoah {fastdebug,release}, tier{1,2} with Shenandoah

-- 
Thanks,
-Aleksey