Squash-to: "mptcp: move drop_other_suboptions check under pm lock"

MPJ ACK is a pure TCP ack on an established TCP connection, so is
eligble to carry MPTCP ADD_ADDR option. If that happens, the MPJ
subopt will be stripped and the MPJ handshake will be broken.

Avoid adding ADD_ADDR subopt if the current packet is MPJ ACK (or
MPC ACK, just to be safe: the added check it's cheap).

This fixes issues/221

Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>

net/mptcp/options.c

@@ -667,7 +667,11 @@ static bool mptcp_established_options_add_addr(struct sock *sk, struct sk_buff *
 	bool port;
 	int len;
 
+	/* add addr will strip the existing options, be sure to avoid breaking
+	 * MPC/MPJ handshakes
+	 */
 	if (!mptcp_pm_should_add_signal(msk) ||
+	    (opts->suboptions & (OPTION_MPTCP_MPJ_ACK | OPTION_MPTCP_MPC_ACK)) ||
 	    !mptcp_pm_add_addr_signal(msk, skb, opt_size, remaining, &opts->addr,
 		    &echo, &port, &drop_other_suboptions))
 		return false;