bcachefs: Fix bch2_journal_flush_device_pins()

If a journal write errored, the list of devices it was written to could
be empty - we're not supposed to mark an empty replicas list.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/journal_reclaim.c

@@ -892,10 +892,12 @@ int bch2_journal_flush_device_pins(struct journal *j, int dev_idx)
 					 journal_seq_pin(j, seq)->devs);
 		seq++;
 
+		if (replicas.e.nr_devs) {
 			spin_unlock(&j->lock);
 			ret = bch2_mark_replicas(c, &replicas.e);
 			spin_lock(&j->lock);
 		}
+	}
 	spin_unlock(&j->lock);
 err:
 	ret = bch2_replicas_gc_end(c, ret);