tcp: Optimise len/mss comparison

Instead of checking len > mss || len == 0, we can accomplish
both by checking (len - 1) > mss using the unsigned wraparound.
At nearly a million times a second, this might just help.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/tcp.c

@@ -2566,7 +2566,7 @@ struct sk_buff **tcp_gro_receive(struct sk_buff **head, struct sk_buff *skb)
 
 	mss = skb_shinfo(p)->gso_size;
 
-	flush |= (len > mss) | !len;
+	flush |= (len - 1) >= mss;
 	flush |= (ntohl(th2->seq) + skb_gro_len(p)) ^ ntohl(th->seq);
 
 	if (flush || skb_gro_receive(head, skb)) {