bcachefs fixes for 6.12

- Assorted tiny syzbot fixes
- Shutdown path fix: "bch2_btree_write_buffer_flush_going_ro()"

  The shutdown path wasn't flushing the btree write buffer, leading to
  shutting down while we still had operations in flight. This fixes a
  whole slew of syzbot bugs, and undoubtedly other strange heisenbugs.
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