RFR: 8215206: VtableStubs::find_stub is not appropriately protected by VtableStubs_lock

[Erik Österlund]

Hi,

In VtableStubs::find_stub we first perform a lookup in the table to 
check for already created stubs under the VtableStubs_lock, then if not 
found we create a new stub without the lock held, and then insert it 
under the VtableStubs_lock without checking for entries inserted between 
the lookup and insertion.
If the VtableStubs_lock is to protect the table (which is the 
intention), the lock should not be unlocked before lookup and insert.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8215206

Webrev:
http://cr.openjdk.java.net/~eosterlund/8215206/webrev.00/

Thanks,
/Erik