RFR: JDK-8260625: Rename MetaspaceExpand_lock

[Thomas Stuefe]

Hi, may I have reviews please for this trivial change:

MetaspaceExpand_lock is a potentially confusing misnomer for the casual code reader, since it does not only guard metaspace expansion but guards access to all central metaspace structures for expansion, reclamation, access to freelist etc. 

With this patch it is renamed to the more generic "Metaspace_lock".

Thanks, Thomas

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Commit messages:
 - Rename lock

Changes: https://git.openjdk.java.net/jdk/pull/2308/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2308&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8260625
  Stats: 71 lines in 10 files changed: 0 ins; 0 del; 71 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2308.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2308/head:pull/2308

PR: https://git.openjdk.java.net/jdk/pull/2308