mptcp: initialize the data_fin field for mpc packets

When parsing MPC+data packets we set the dss field, so
we must also initialize the data_fin, or we can find stray
value there.

Fixes: 9a19371b ("mptcp: fix data_fin handing in RX path")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -858,6 +858,7 @@ void mptcp_incoming_options(struct sock *sk, struct sk_buff *skb,
 			mpext->subflow_seq = 1;
 			mpext->dsn64 = 1;
 			mpext->mpc_map = 1;
+			mpext->data_fin = 0;
 		} else {
 			mpext->data_seq = mp_opt.data_seq;
 			mpext->subflow_seq = mp_opt.subflow_seq;