bcachefs: Fix error reporting from bch2_journal_flush_seq

- bch2_journal_halt() was unconditionally overwriting j->err_seq, the
  sequence number that we failed to write
- journal_write_done was updating seq_ondisk and flushed_seq_ondisk even
  for writes that errored, which broke the way bch2_journal_flush_seq_async()
  locklessly checked for completions.

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>