net: ipv4: Refine the ipv4_default_advmss

1. Don't get the metric RTAX_ADVMSS of dst.
There are two reasons.
1) Its caller dst_metric_advmss has already invoke dst_metric_advmss
before invoke default_advmss.
2) The ipv4_default_advmss is used to get the default mss, it should
not try to get the metric like ip6_default_advmss.

2. Use sizeof(tcphdr)+sizeof(iphdr) instead of literal 40.

3. Define one new macro IPV4_MAX_PMTU instead of 65535 according to
RFC 2675, section 5.1.
Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/ip.h

@@ -33,6 +33,8 @@
 #include <net/flow.h>
 #include <net/flow_dissector.h>
 
+#define IPV4_MAX_PMTU		65535U		/* RFC 2675, Section 5.1 */
+
 struct sock;
 
 struct inet_skb_parm {

net/ipv4/route.c

@@ -1250,15 +1250,11 @@ static void set_class_tag(struct rtable *rt, u32 tag)
 
 static unsigned int ipv4_default_advmss(const struct dst_entry *dst)
 {
-	unsigned int advmss = dst_metric_raw(dst, RTAX_ADVMSS);
+	unsigned int header_size = sizeof(struct tcphdr) + sizeof(struct iphdr);
+	unsigned int advmss = max_t(unsigned int, dst->dev->mtu - header_size,
+				    ip_rt_min_advmss);
 
-	if (advmss == 0) {
-		advmss = max_t(unsigned int, dst->dev->mtu - 40,
-			       ip_rt_min_advmss);
-		if (advmss > 65535 - 40)
-			advmss = 65535 - 40;
-	}
-	return advmss;
+	return min(advmss, IPV4_MAX_PMTU - header_size);
 }
 
 static unsigned int ipv4_mtu(const struct dst_entry *dst)