bcachefs fixes for 6.12-rc7

Some trivial syzbot fixes, two more serious btree fixes found by looping
single_devices.ktest small_nodes:

- Topology error on split after merge, where we accidentaly picked the
  node being deleted for the pivot, resulting in an assertion pop

- New nodes being preallocated were left on the freedlist, unlocked,
  resulting in them sometimes being accidentally freed: this dated from
  pre-cycle detector, when we could leave them locked. This should have
  resulted in more explosions and fireworks, but turned out to be
  surprisingly hard to hit because the preallocated nodes were being
  used right away.

  the fix for this is bigger than we'd like - reworking btree list
  handling was a bit invasive - but we've now got more assertions and
  it's well tested.

- Also another mishandled transaction restart fix (in
  btree_node_prefetch) - we're almost done with those.
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