bcachefs fixes for 6.12-rc6

Various syzbot fixes, and the more notable ones:

- Fix for pointers in an extent overflowing the max (16) on a filesystem
  with many devices: we were creating too many cached copies when moving
  data around. Now, we only create at most one cached copy if there's a
  promote target set.

  Caching will be a bit broken for reflinked data until 6.13: I have
  larger series queued up which significantly improves the plumbing for
  data options down into the extent (bch_extent_rebalance) to fix this.

- Fix for deadlock on -ENOSPC on tiny filesystems

  Allocation from the partial open_bucket list wasn't correctly
  accounting partial open_buckets as free: this fixes the main cause of
  tests timing out in the automated tests.
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