net: ipv4: Don't crash if passing a null sk to ip_do_redirect.

Commit e2d118a1 ("net: inet: Support UID-based routing in IP
protocols.") made ip_do_redirect call sock_net(sk) to determine
the network namespace of the passed-in socket. This crashes if sk
is NULL.

Fix this by getting the network namespace from the skb instead.

Fixes: e2d118a1 ("net: inet: Support UID-based routing in IP protocols.")
Signed-off-by: NLorenzo Colitti <lorenzo@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv4/route.c

@@ -798,6 +798,7 @@ static void ip_do_redirect(struct dst_entry *dst, struct sock *sk, struct sk_buf
 	struct rtable *rt;
 	struct flowi4 fl4;
 	const struct iphdr *iph = (const struct iphdr *) skb->data;
+	struct net *net = dev_net(skb->dev);
 	int oif = skb->dev->ifindex;
 	u8 tos = RT_TOS(iph->tos);
 	u8 prot = iph->protocol;
@@ -805,7 +806,7 @@ static void ip_do_redirect(struct dst_entry *dst, struct sock *sk, struct sk_buf
 
 	rt = (struct rtable *) dst;
 
-	__build_flow_key(sock_net(sk), &fl4, sk, iph, oif, tos, prot, mark, 0);
+	__build_flow_key(net, &fl4, sk, iph, oif, tos, prot, mark, 0);
 	__ip_do_redirect(rt, skb, &fl4, true);
 }