RFR 8182720: In InstanceKlass.cpp, get init_lock from handle

[harold seigel]

Hi,

Please review this small fix to instanceKlass.cpp to get the init_lock 
from its handle instead of looking in the klass mirror. This change was 
missed when fixing JDK8176705 
<https://bugs.openjdk.java.net/browse/JDK-8176705>.

Open Webrev: 
http://cr.openjdk.java.net/~hseigel/bug_8182720/webrev/index.html

JBS Bug:  https://bugs.openjdk.java.net/browse/JDK-8182720

The fix was tested with JCK Lang and VM tests, the JTreg hotspot, 
java/io, java/lang, java/util and other tests, the co-located NSK tests, 
and with JPRT.

Thanks, Harold