packet: fix match_fanout_group()

Recent TCP listener patches exposed a prior af_packet bug :
match_fanout_group() blindly assumes it is always safe
to cast sk to a packet socket to compare fanout with af_packet_priv

But SYNACK packets can be sent while attached to request_sock, which
are smaller than a "struct sock".

We can read non existent memory and crash.

Fixes: c0de08d ("af_packet: don't emit packet on orig fanout group")
Fixes: ca6fb06 ("tcp: attach SYNACK messages to request sockets instead of listener")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Eric Leblond <eric@regit.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/packet/af_packet.c

@@ -1519,10 +1519,10 @@ static void __fanout_unlink(struct sock *sk, struct packet_sock *po)
 
 static bool match_fanout_group(struct packet_type *ptype, struct sock *sk)
 {
-	if (ptype->af_packet_priv == (void *)((struct packet_sock *)sk)->fanout)
-		return true;
+	if (sk->sk_family != PF_PACKET)
+		return false;
 
-	return false;
+	return ptype->af_packet_priv == pkt_sk(sk)->fanout;
 }
 
 static void fanout_init_data(struct packet_fanout *f)