[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/addrlabel.c b/net/ipv6/addrlabel.c
index de371b5..9bfa884 100644
--- a/net/ipv6/addrlabel.c
+++ b/net/ipv6/addrlabel.c
@@ -364,7 +364,7 @@
 static int ip6addrlbl_newdel(struct sk_buff *skb, struct nlmsghdr *nlh,
 			     void *arg)
 {
-	struct net *net = skb->sk->sk_net;
+	struct net *net = sock_net(skb->sk);
 	struct ifaddrlblmsg *ifal;
 	struct nlattr *tb[IFAL_MAX+1];
 	struct in6_addr *pfx;
@@ -452,7 +452,7 @@
 
 static int ip6addrlbl_dump(struct sk_buff *skb, struct netlink_callback *cb)
 {
-	struct net *net = skb->sk->sk_net;
+	struct net *net = sock_net(skb->sk);
 	struct ip6addrlbl_entry *p;
 	struct hlist_node *pos;
 	int idx = 0, s_idx = cb->args[0];
@@ -490,7 +490,7 @@
 static int ip6addrlbl_get(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 ifaddrlblmsg *ifal;
 	struct nlattr *tb[IFAL_MAX+1];
 	struct in6_addr *addr;