mptcp: fix data_fin handing in RX path

The data fin flag is set only via a DSS option, but
mptcp_incoming_options() copies it unconditionally from the
provided RX options.

Since we do not clear all the mptcp sock RX options in a
socket free/alloc cycle, we can end-up with a stray data_fin
value while parsing e.g. MPC packets.

That would lead to mapping data corruption and will trigger
a few WARN_ON() in the RX path.

Instead of adding a costly memset(), fetch the data_fin flag
only for DSS packets - when we always explicitly initialize
such bit at option parsing time.

Fixes: 648ef4b8 ("mptcp: Implement MPTCP receive path")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/mptcp/options.c

@@ -876,12 +876,11 @@ void mptcp_incoming_options(struct sock *sk, struct sk_buff *skb,
 			mpext->data_seq = mp_opt->data_seq;
 			mpext->subflow_seq = mp_opt->subflow_seq;
 			mpext->dsn64 = mp_opt->dsn64;
+			mpext->data_fin = mp_opt->data_fin;
 		}
 		mpext->data_len = mp_opt->data_len;
 		mpext->use_map = 1;
 	}
-
-	mpext->data_fin = mp_opt->data_fin;
 }
 
 void mptcp_write_options(__be32 *ptr, struct mptcp_out_options *opts)