jbd: don't write superblock when unmounting an ro filesystem

This sequence:

results in an IO error when unmounting the RO filesystem. The bug was
introduced by:

commit 9754e39c
Author: Jan Kara <jack@suse.cz>
Date:   Sat Apr 7 12:33:03 2012 +0200

    jbd: Split updating of journal superblock and marking journal empty

which lost some of the magic in journal_update_superblock() which
used to test for a journal with no outstanding transactions.

This is a port of a jbd2 fix by Eric Sandeen.

CC: <stable@vger.kernel.org> # 3.4.x
Signed-off-by: Jan Kara <jack@suse.cz>

fs/jbd/journal.c

@@ -1113,6 +1113,11 @@ static void mark_journal_empty(journal_t *journal)
 
 	BUG_ON(!mutex_is_locked(&journal->j_checkpoint_mutex));
 	spin_lock(&journal->j_state_lock);
+	/* Is it already empty? */
+	if (sb->s_start == 0) {
+		spin_unlock(&journal->j_state_lock);
+		return;
+	}
 	jbd_debug(1, "JBD: Marking journal as empty (seq %d)\n",
         	  journal->j_tail_sequence);