tcp: SYN packets are now simply consumed

We now have proper per-listener but also per network namespace counters
for SYN packets that might be dropped.

We replace the kfree_skb() by consume_skb() to be drop monitor [1]
friendly, and remove an obsolete comment.
FastOpen SYN packets can carry payload in them just fine.

[1] perf record -a -g -e skb:kfree_skb sleep 1; perf report

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: edumazet

net/ipv4/tcp_input.c

@@ -5815,24 +5815,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb)
 			if (icsk->icsk_af_ops->conn_request(sk, skb) < 0)
 				return 1;
 
-			/* Now we have several options: In theory there is
-			 * nothing else in the frame. KA9Q has an option to
-			 * send data with the syn, BSD accepts data with the
-			 * syn up to the [to be] advertised window and
-			 * Solaris 2.1 gives you a protocol error. For now
-			 * we just ignore it, that fits the spec precisely
-			 * and avoids incompatibilities. It would be nice in
-			 * future to drop through and process the data.
-			 *
-			 * Now that TTCP is starting to be used we ought to
-			 * queue this data.
-			 * But, this leaves one open to an easy denial of
-			 * service attack, and SYN cookies can't defend
-			 * against this problem. So, we drop the data
-			 * in the interest of security over speed unless
-			 * it's still in use.
-			 */
-			kfree_skb(skb);
+			consume_skb(skb);
 			return 0;
 		}
 		goto discard;