bcachefs: Fix bch2_mark_extent()

If an extent only contained cached or erasure coded pointers, there
won't be any devices in the normal dirty replicas list or an entry to
update.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/buckets.c

@@ -1063,7 +1063,8 @@ static int bch2_mark_extent(struct bch_fs *c, struct bkey_s_c k,
 		}
 	}
 
-	update_replicas(c, fs_usage, &r.e, dirty_sectors);
+	if (r.e.nr_devs)
+		update_replicas(c, fs_usage, &r.e, dirty_sectors);
 
 	return 0;
 }