RFR: 8304738: UnregisteredClassesTable_lock never created [v2]

[Matias Saavedra Silva]

> UnregisteredClassesTable_lock is never created in mutex_init() and is always nullptr. This lock is referenced in a few places and all of those are effectively thread unsafe. Some of the places `unregistered_classes_table` is accessed are under other locks, so the rank of this lock has to be adjusted appropriately. The lock will be initialized to enable the intended behavior. Verified with tier 1-4 tests.

Matias Saavedra Silva has updated the pull request incrementally with one additional commit since the last revision:

  Moved mutex up before read

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Changes:
  - all: https://git.openjdk.org/jdk/pull/13358/files
  - new: https://git.openjdk.org/jdk/pull/13358/files/312e726b..be1a754a

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13358&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13358&range=00-01

  Stats: 12 lines in 1 file changed: 5 ins; 3 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/13358.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13358/head:pull/13358

PR: https://git.openjdk.org/jdk/pull/13358