bcachefs: Fix failure to return error in data_update_index_update()

This fixes an assertion pop in io_write.c - if we don't return an error
we're supposed to have completed all the btree updates.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/data_update.c

@@ -337,6 +337,7 @@ static int __bch2_data_update_index_update(struct btree_trans *trans,
 			printbuf_exit(&buf);
 
 			bch2_fatal_error(c);
+			ret = -EIO;
 			goto out;
 		}