udp: Add udp6_lib_lookup_skb and udp4_lib_lookup_skb

Add externally visible functions to lookup a UDP socket by skb. This
will be used for GRO in UDP sockets. These functions also check
if skb->dst is set, and if it is not skb->dev is used to get dev_net.
This allows calling lookup functions before dst has been set on the
skbuff.

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/udp.c

@@ -604,6 +604,19 @@ static inline struct sock *__udp4_lib_lookup_skb(struct sk_buff *skb,
 				 udptable, skb);
 }
 
+struct sock *udp4_lib_lookup_skb(struct sk_buff *skb,
+				 __be16 sport, __be16 dport)
+{
+	const struct iphdr *iph = ip_hdr(skb);
+	const struct net_device *dev =
+	    skb_dst(skb) ? skb_dst(skb)->dev : skb->dev;
+
+	return __udp4_lib_lookup(dev_net(dev), iph->saddr, sport,
+				 iph->daddr, dport, inet_iif(skb),
+				 &udp_table, skb);
+}
+EXPORT_SYMBOL_GPL(udp4_lib_lookup_skb);
+
 /* Must be called under rcu_read_lock().
  * Does increment socket refcount.
  */