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>

fs/bcachefs/journal.c

@@ -716,7 +716,7 @@ int bch2_journal_flush_seq_async(struct journal *j, u64 seq,
 			return ret;
 
 		seq = res.seq;
-		buf = j->buf + (seq & JOURNAL_BUF_MASK);
+		buf = journal_seq_to_buf(j, seq);
 		buf->must_flush = true;
 
 		if (!buf->flush_time) {
@@ -734,8 +734,8 @@ int bch2_journal_flush_seq_async(struct journal *j, u64 seq,
 	}
 
 	/*
-	 * if write was kicked off without a flush, flush the next sequence
-	 * number instead
+	 * if write was kicked off without a flush, or if we promised it
+	 * wouldn't be a flush, flush the next sequence number instead
 	 */
 	buf = journal_seq_to_buf(j, seq);
 	if (buf->noflush) {
@@ -813,8 +813,8 @@ bool bch2_journal_noflush_seq(struct journal *j, u64 seq)
 	     unwritten_seq++) {
 		struct journal_buf *buf = journal_seq_to_buf(j, unwritten_seq);
 
-		/* journal write is already in flight, and was a flush write: */
-		if (unwritten_seq == journal_last_unwritten_seq(j) && !buf->noflush)
+		/* journal flush already in flight, or flush requseted */
+		if (buf->must_flush)
 			goto out;
 
 		buf->noflush = true;

fs/bcachefs/journal_io.c

@@ -1936,6 +1936,7 @@ static int bch2_journal_write_pick_flush(struct journal *j, struct journal_buf *
 
 		j->nr_noflush_writes++;
 	} else {
+		w->must_flush = true;
 		j->last_flush_write = jiffies;
 		j->nr_flush_writes++;
 		clear_bit(JOURNAL_NEED_FLUSH_WRITE, &j->flags);