ipv4: Only compute net once in ip_call_ra_chain

ip_call_ra_chain is called early in the forwarding chain from
ip_forward and ip_mr_input, which makes skb->dev the correct
expression to get the input network device and dev_net(skb->dev) a
correct expression for the network namespace the packet is being
processed in.

Compute the network namespace and store it in a variable to make the
code clearer.
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/ip_input.c

@@ -157,6 +157,7 @@ bool ip_call_ra_chain(struct sk_buff *skb)
 	u8 protocol = ip_hdr(skb)->protocol;
 	struct sock *last = NULL;
 	struct net_device *dev = skb->dev;
+	struct net *net = dev_net(dev);
 
 	for (ra = rcu_dereference(ip_ra_chain); ra; ra = rcu_dereference(ra->next)) {
 		struct sock *sk = ra->sk;
@@ -167,7 +168,7 @@ bool ip_call_ra_chain(struct sk_buff *skb)
 		if (sk && inet_sk(sk)->inet_num == protocol &&
 		    (!sk->sk_bound_dev_if ||
 		     sk->sk_bound_dev_if == dev->ifindex) &&
-		    net_eq(sock_net(sk), dev_net(dev))) {
+		    net_eq(sock_net(sk), net)) {
 			if (ip_is_fragment(ip_hdr(skb))) {
 				if (ip_defrag(skb, IP_DEFRAG_CALL_RA_CHAIN))
 					return true;