l2tp: WARN_ON rather than BUG_ON in l2tp_dfs_seq_start

l2tp_dfs_seq_start had a BUG_ON to catch a possible programming error in
l2tp_dfs_seq_open.

Since we can easily bail out of l2tp_dfs_seq_start, prefer to do that
and flag the error with a WARN_ON rather than crashing the kernel.
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_debugfs.c

@@ -72,7 +72,10 @@ static void *l2tp_dfs_seq_start(struct seq_file *m, loff_t *offs)
 	if (!pos)
 		goto out;
 
-	BUG_ON(!m->private);
+	if (WARN_ON(!m->private)) {
+		pd = NULL;
+		goto out;
+	}
 	pd = m->private;
 
 	if (!pd->tunnel)