RFR: 8280830: Change NonblockingQueue::try_pop variable named "result"

[Kim Barrett]

Please review this trivial change to rename a variable in
NonblockingQueue::try_pop.  The variable named "result" is being renamed to
"old_head", as the old name was found to be confusing by some people, making
the code harder to read.

Testing:
local build.

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Commit messages:
 - change variable name

Changes: https://git.openjdk.java.net/jdk/pull/7394/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7394&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280830
  Stats: 24 lines in 1 file changed: 0 ins; 0 del; 24 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7394.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7394/head:pull/7394

PR: https://git.openjdk.java.net/jdk/pull/7394