syncookies: avoid unneeded tcp header flag double check

caller: if (!th->rst && !th->syn && th->ack)
callee: if (!th->ack)

make the caller only check for !syn (common for 3whs), and move
the !rst / ack test to the callee.
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/syncookies.c

@@ -266,7 +266,7 @@ struct sock *cookie_v4_check(struct sock *sk, struct sk_buff *skb,
 	struct rtable *rt;
 	__u8 rcv_wscale;
 
-	if (!sysctl_tcp_syncookies || !th->ack)
+	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 		goto out;
 
 	if (tcp_synq_no_recent_overflow(sk) ||

net/ipv4/tcp_ipv4.c

@@ -1506,7 +1506,7 @@ static struct sock *tcp_v4_hnd_req(struct sock *sk, struct sk_buff *skb)
 	}
 
 #ifdef CONFIG_SYN_COOKIES
-	if (!th->rst && !th->syn && th->ack)
+	if (!th->syn)
 		sk = cookie_v4_check(sk, skb, &(IPCB(skb)->opt));
 #endif
 	return sk;

net/ipv6/syncookies.c

@@ -174,7 +174,7 @@ struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb)
 	struct dst_entry *dst;
 	__u8 rcv_wscale;
 
-	if (!sysctl_tcp_syncookies || !th->ack)
+	if (!sysctl_tcp_syncookies || !th->ack || th->rst)
 		goto out;
 
 	if (tcp_synq_no_recent_overflow(sk) ||

net/ipv6/tcp_ipv6.c

@@ -1157,7 +1157,7 @@ static struct sock *tcp_v6_hnd_req(struct sock *sk,struct sk_buff *skb)
 	}
 
 #ifdef CONFIG_SYN_COOKIES
-	if (!th->rst && !th->syn && th->ack)
+	if (!th->syn)
 		sk = cookie_v6_check(sk, skb);
 #endif
 	return sk;