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>
diff --git a/include/net/ip.h b/include/net/ip.h
index bf264a8..821cedc 100644
--- a/include/net/ip.h
+++ b/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 {