mptcp: keep snd_una updated for fallback socket

After shutdown, for fallback MPTCP sockets, we always have

write_seq == snd_una+1

The above will foul OUTQ ioctl(). Keep snd_una in sync with
write_seq even after shutdown.
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/protocol.c

@@ -2666,6 +2666,7 @@ static void __mptcp_check_send_data_fin(struct sock *sk)
 	 * state now
 	 */
 	if (__mptcp_check_fallback(msk)) {
+		WRITE_ONCE(msk->snd_una, msk->write_seq);
 		if ((1 << sk->sk_state) & (TCPF_CLOSING | TCPF_LAST_ACK)) {
 			inet_sk_state_store(sk, TCP_CLOSE);
 			mptcp_close_wake_up(sk);