bcachefs: Unwritten journal buffers are always dirty

Ensure that journal bufs that haven't been written can't be reclaimed
from the journal pin fifo, and can thus have new pins taken.

Prep work for changing the btree write buffer to pull keys from the
journal directly.

Signed-off-by: Kent Overstreet <[e-mail]>

fs/bcachefs/journal_reclaim.c

···
 	 * all btree nodes got written out
 	 */
 	while (!fifo_empty(&j->pin) &&
+	       j->pin.front <= j->seq_ondisk &&
 	       !atomic_read(&fifo_peek_front(&j->pin).count)) {
 		j->pin.front++;
 		popped = true;