RFR: 8306767: Concurrent repacking of extra data in MethodData is potentially unsafe [v2]

[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:

  adding more verification and more locking, WIP

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/16840/files
  - new: https://git.openjdk.org/jdk/pull/16840/files/9336c4fe..5b819f19

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=16840&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=16840&range=00-01

  Stats: 103 lines in 9 files changed: 81 ins; 12 del; 10 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