[NET] NETNS: Omit net_device->nd_net without CONFIG_NET_NS.

Introduce per-net_device inlines: dev_net(), dev_net_set().
Without CONFIG_NET_NS, no namespace other than &init_net exists.
Let's explicitly define them to help compiler optimizations.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>

net/ipv6/ip6_output.c

@@ -402,7 +402,7 @@ int ip6_forward(struct sk_buff *skb)
 	struct dst_entry *dst = skb->dst;
 	struct ipv6hdr *hdr = ipv6_hdr(skb);
 	struct inet6_skb_parm *opt = IP6CB(skb);
-	struct net *net = dst->dev->nd_net;
+	struct net *net = dev_net(dst->dev);
 
 	if (ipv6_devconf.forwarding == 0)
 		goto error;