mptcp: drop all sub-options except ADD_ADDR when the echo bit is set

Current Linux carries echo-ed ADD_ADDR over pure TCP ACKs, so there is no
need to add a DSS element that would fit only ADD_ADDR with IPv4 address.
Drop the DSS from echo-ed ADD_ADDR, regardless of the IP version.
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -624,7 +624,8 @@ static bool mptcp_established_options_add_addr(struct sock *sk, struct sk_buff *
 	int len;
 
 	if ((mptcp_pm_should_add_signal_ipv6(msk) ||
-	     mptcp_pm_should_add_signal_port(msk)) &&
+	     mptcp_pm_should_add_signal_port(msk) ||
+	     mptcp_pm_should_add_signal_echo(msk)) &&
 	    skb && skb_is_tcp_pure_ack(skb)) {
 		pr_debug("drop other suboptions");
 		opts->suboptions = 0;