[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/af_inet6.c b/net/ipv6/af_inet6.c
index f52bdae..12f04e9 100644
--- a/net/ipv6/af_inet6.c
+++ b/net/ipv6/af_inet6.c
@@ -245,7 +245,7 @@
 	struct sock *sk = sock->sk;
 	struct inet_sock *inet = inet_sk(sk);
 	struct ipv6_pinfo *np = inet6_sk(sk);
-	struct net *net = sk->sk_net;
+	struct net *net = sock_net(sk);
 	__be32 v4addr = 0;
 	unsigned short snum;
 	int addr_type = 0;
@@ -438,7 +438,7 @@
 int inet6_ioctl(struct socket *sock, unsigned int cmd, unsigned long arg)
 {
 	struct sock *sk = sock->sk;
-	struct net *net = sk->sk_net;
+	struct net *net = sock_net(sk);
 
 	switch(cmd)
 	{