[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/ipv4/igmp.c b/net/ipv4/igmp.c
index 682f632..6250f42 100644
--- a/net/ipv4/igmp.c
+++ b/net/ipv4/igmp.c
@@ -1762,7 +1762,7 @@
 	if (!ipv4_is_multicast(addr))
 		return -EINVAL;
 
-	if (sk->sk_net != &init_net)
+	if (sock_net(sk) != &init_net)
 		return -EPROTONOSUPPORT;
 
 	rtnl_lock();
@@ -1833,7 +1833,7 @@
 	u32 ifindex;
 	int ret = -EADDRNOTAVAIL;
 
-	if (sk->sk_net != &init_net)
+	if (sock_net(sk) != &init_net)
 		return -EPROTONOSUPPORT;
 
 	rtnl_lock();
@@ -1881,7 +1881,7 @@
 	if (!ipv4_is_multicast(addr))
 		return -EINVAL;
 
-	if (sk->sk_net != &init_net)
+	if (sock_net(sk) != &init_net)
 		return -EPROTONOSUPPORT;
 
 	rtnl_lock();
@@ -2017,7 +2017,7 @@
 	    msf->imsf_fmode != MCAST_EXCLUDE)
 		return -EINVAL;
 
-	if (sk->sk_net != &init_net)
+	if (sock_net(sk) != &init_net)
 		return -EPROTONOSUPPORT;
 
 	rtnl_lock();
@@ -2100,7 +2100,7 @@
 	if (!ipv4_is_multicast(addr))
 		return -EINVAL;
 
-	if (sk->sk_net != &init_net)
+	if (sock_net(sk) != &init_net)
 		return -EPROTONOSUPPORT;
 
 	rtnl_lock();
@@ -2165,7 +2165,7 @@
 	if (!ipv4_is_multicast(addr))
 		return -EINVAL;
 
-	if (sk->sk_net != &init_net)
+	if (sock_net(sk) != &init_net)
 		return -EPROTONOSUPPORT;
 
 	rtnl_lock();
@@ -2252,7 +2252,7 @@
 	if (inet->mc_list == NULL)
 		return;
 
-	if (sk->sk_net != &init_net)
+	if (sock_net(sk) != &init_net)
 		return;
 
 	rtnl_lock();