More new code for 5.12:

- Restore a disused sysctl control knob that was inadvertently dropped
  during the merge window to avoid fstests regressions.
- Don't speculatively release freed blocks from the busy list until
  we're actually allocating them, which fixes a rare log recovery
  regression.
- Don't nest transactions when scanning for free space.
- Add an idiot^Wmaintainer light to detect nested transactions. ;)
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