ipv4: ip_tunnel: use net namespace from rtable not socket

The socket parameter might legally be NULL, thus sock_net is sometimes
causing a NULL pointer dereference. Using net_device pointer in dst_entry
is more reliable.

Fixes: b6a7719 ("ipv4: hash net ptr into fragmentation bucket selection")
Reported-by: Rick Jones <rick.jones2@hp.com>
Cc: Rick Jones <rick.jones2@hp.com>
Cc: David S. Miller <davem@davemloft.net>
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/ip_tunnel_core.c

@@ -74,7 +74,8 @@ int iptunnel_xmit(struct sock *sk, struct rtable *rt, struct sk_buff *skb,
 	iph->daddr	=	dst;
 	iph->saddr	=	src;
 	iph->ttl	=	ttl;
-	__ip_select_ident(sock_net(sk), iph, skb_shinfo(skb)->gso_segs ?: 1);
+	__ip_select_ident(dev_net(rt->dst.dev), iph,
+			  skb_shinfo(skb)->gso_segs ?: 1);
 
 	err = ip_local_out_sk(sk, skb);
 	if (unlikely(net_xmit_eval(err)))