[NET] NETNS: Omit sock->sk_net without CONFIG_NET_NS.

Introduce per-sock inlines: sock_net(), sock_net_set()
and per-inet_timewait_sock inlines: twsk_net(), twsk_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>
diff --git a/net/ipv6/route.c b/net/ipv6/route.c
index 65053fb..ac44283 100644
--- a/net/ipv6/route.c
+++ b/net/ipv6/route.c
@@ -2020,7 +2020,7 @@
 
 	cfg->fc_nlinfo.pid = NETLINK_CB(skb).pid;
 	cfg->fc_nlinfo.nlh = nlh;
-	cfg->fc_nlinfo.nl_net = skb->sk->sk_net;
+	cfg->fc_nlinfo.nl_net = sock_net(skb->sk);
 
 	if (tb[RTA_GATEWAY]) {
 		nla_memcpy(&cfg->fc_gateway, tb[RTA_GATEWAY], 16);
@@ -2216,7 +2216,7 @@
 
 static int inet6_rtm_getroute(struct sk_buff *in_skb, struct nlmsghdr* nlh, void *arg)
 {
-	struct net *net = in_skb->sk->sk_net;
+	struct net *net = sock_net(in_skb->sk);
 	struct nlattr *tb[RTA_MAX+1];
 	struct rt6_info *rt;
 	struct sk_buff *skb;