[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/rose/af_rose.c b/net/rose/af_rose.c
index 1a7f143..92d85c3 100644
--- a/net/rose/af_rose.c
+++ b/net/rose/af_rose.c
@@ -551,7 +551,7 @@
 	if (osk->sk_type != SOCK_SEQPACKET)
 		return NULL;
 
-	sk = sk_alloc(osk->sk_net, PF_ROSE, GFP_ATOMIC, &rose_proto);
+	sk = sk_alloc(sock_net(osk), PF_ROSE, GFP_ATOMIC, &rose_proto);
 	if (sk == NULL)
 		return NULL;