[11u] RFR 8237508: Simplify JarFile.isInitializing

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8237508
  https://hg.openjdk.java.net/jdk/jdk/rev/c5203b158ec8

This is the follow up after JDK-8234466. Patch does not apply cleanly to 11u, because of minor
context changes.

11u webrev:
  https://cr.openjdk.java.net/~shade/8237508/webrev.11u.01/

Testing: tier1, tier2, tier3

-- 
Thanks,
-Aleksey

[1] https://bugs.openjdk.java.net/browse/JDK-8234466