net: ipv4: avoid repeated calls to ip_skb_dst_mtu helper

ip_skb_dst_mtu is small inline helper, but its called in several places.

before: 17061      44       0   17105    42d1 net/ipv4/ip_output.o
after:  16805      44       0   16849    41d1 net/ipv4/ip_output.o

Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_output.c

@@ -84,6 +84,7 @@ int sysctl_ip_default_ttl __read_mostly = IPDEFTTL;
 EXPORT_SYMBOL(sysctl_ip_default_ttl);
 
 static int ip_fragment(struct sock *sk, struct sk_buff *skb,
+		       unsigned int mtu,
 		       int (*output)(struct sock *, struct sk_buff *));
 
 /* Generate a checksum for an outgoing IP datagram. */
@@ -219,15 +220,16 @@ static inline int ip_finish_output2(struct sock *sk, struct sk_buff *skb)
 	return -EINVAL;
 }
 
-static int ip_finish_output_gso(struct sock *sk, struct sk_buff *skb)
+static int ip_finish_output_gso(struct sock *sk, struct sk_buff *skb,
+				unsigned int mtu)
 {
 	netdev_features_t features;
 	struct sk_buff *segs;
 	int ret = 0;
 
 	/* common case: locally created skb or seglen is <= mtu */
 	if (((IPCB(skb)->flags & IPSKB_FORWARDED) == 0) ||
-	      skb_gso_network_seglen(skb) <= ip_skb_dst_mtu(skb))
+	      skb_gso_network_seglen(skb) <= mtu)
 		return ip_finish_output2(sk, skb);
 
 	/* Slowpath -  GSO segment length is exceeding the dst MTU.
@@ -251,7 +253,7 @@ static int ip_finish_output_gso(struct sock *sk, struct sk_buff *skb)
 		int err;
 
 		segs->next = NULL;
-		err = ip_fragment(sk, segs, ip_finish_output2);
+		err = ip_fragment(sk, segs, mtu, ip_finish_output2);
 
 		if (err && ret == 0)
 			ret = err;
@@ -263,18 +265,21 @@ static int ip_finish_output_gso(struct sock *sk, struct sk_buff *skb)
 
 static int ip_finish_output(struct sock *sk, struct sk_buff *skb)
 {
+	unsigned int mtu;
+
 #if defined(CONFIG_NETFILTER) && defined(CONFIG_XFRM)
 	/* Policy lookup after SNAT yielded a new policy */
 	if (skb_dst(skb)->xfrm) {
 		IPCB(skb)->flags |= IPSKB_REROUTED;
 		return dst_output_sk(sk, skb);
 	}
 #endif
+	mtu = ip_skb_dst_mtu(skb);
 	if (skb_is_gso(skb))
-		return ip_finish_output_gso(sk, skb);
+		return ip_finish_output_gso(sk, skb, mtu);
 
-	if (skb->len > ip_skb_dst_mtu(skb))
-		return ip_fragment(sk, skb, ip_finish_output2);
+	if (skb->len > mtu)
+		return ip_fragment(sk, skb, mtu, ip_finish_output2);
 
 	return ip_finish_output2(sk, skb);
 }
@@ -482,10 +487,10 @@ static void ip_copy_metadata(struct sk_buff *to, struct sk_buff *from)
 }
 
 static int ip_fragment(struct sock *sk, struct sk_buff *skb,
+		       unsigned int mtu,
 		       int (*output)(struct sock *, struct sk_buff *))
 {
 	struct iphdr *iph = ip_hdr(skb);
-	unsigned int mtu = ip_skb_dst_mtu(skb);
 
 	if (unlikely(((iph->frag_off & htons(IP_DF)) && !skb->ignore_df) ||
 		     (IPCB(skb)->frag_max_size &&