[NET] NETNS: Omit net_device->nd_net without CONFIG_NET_NS.

Introduce per-net_device inlines: dev_net(), dev_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/sctp/protocol.c b/net/sctp/protocol.c
index beea2fb..2faa0d8 100644
--- a/net/sctp/protocol.c
+++ b/net/sctp/protocol.c
@@ -630,7 +630,7 @@
 	struct sctp_sockaddr_entry *temp;
 	int found = 0;
 
-	if (ifa->ifa_dev->dev->nd_net != &init_net)
+	if (dev_net(ifa->ifa_dev->dev) != &init_net)
 		return NOTIFY_DONE;
 
 	switch (ev) {