RFR: 8214144: Remove confusing locking_enqueue_completed_buffer

[Kim Barrett]

Please review this trivial change to remove the confusingly named
function PtrQueue::locking_enqueue_completed_buffer, inlining its
simple definition at the single call site.  It used to be more
complicated and "interesting", but was gutted by JDK-8182703.

CR:
https://bugs.openjdk.java.net/browse/JDK-8214144

Webrev:
http://cr.openjdk.java.net/~kbarrett/8214144/open.00/

Testing:
Locally (linux-x64) built and ran hotspot_tier1 tests.