RFR: 8306767: Concurrent repacking of extra data in MethodData is potentially unsafe [v3]

[Emanuel Peter]

> I'm making sure that `allocate_bci_to_data` is only called when holding the `extra_data_lock`, so that no concurrent calls of it can ever occur.
> 
> Testing: tier1-3 and stress.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  more locking, still fails tho - WIP

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Changes:
  - all: https://git.openjdk.org/jdk/pull/16840/files
  - new: https://git.openjdk.org/jdk/pull/16840/files/5b819f19..54f2c498

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16840&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16840&range=01-02

  Stats: 19 lines in 3 files changed: 15 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/16840.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16840/head:pull/16840

PR: https://git.openjdk.org/jdk/pull/16840