l2tp: don't use tunnel socket sk_user_data in ppp procfs output

l2tp's ppp procfs output can be used to show internal state of
pppol2tp. It includes a 'user-data-ok' field, which is derived from
the tunnel socket's sk_user_data being non-NULL. Use tunnel->sock
being non-NULL to indicate this instead.
Signed-off-by: James Chapman <jchapman@katalix.com>
Signed-off-by: Tom Parkin <tparkin@katalix.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/l2tp/l2tp_ppp.c

@@ -1513,7 +1513,7 @@ static void pppol2tp_seq_tunnel_show(struct seq_file *m, void *v)
 
 	seq_printf(m, "\nTUNNEL '%s', %c %d\n",
 		   tunnel->name,
-		   (tunnel == tunnel->sock->sk_user_data) ? 'Y' : 'N',
+		   tunnel->sock ? 'Y' : 'N',
 		   refcount_read(&tunnel->ref_count) - 1);
 	seq_printf(m, " %08x %ld/%ld/%ld %ld/%ld/%ld\n",
 		   0,