bcachefs: Fix bch2_journal_noflush_seq()

Improved journal pipelining broke journal_noflush_seq(); it implicitly
assumed only the oldest outstanding journal buf could be in flight, but
that's no longer true.

Make this more straightforward by just setting buf->must_flush whenever
we know a journal buf is going to be flush.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>