netns: Deduplicate and fix copy_net_ns when !CONFIG_NET_NS

The copy of copy_net_ns used when the network stack is not
built is broken as it does not return -EINVAL when attempting
to create a new network namespace.  We don't even have
a previous network namespace.

Since we need a copy of copy_net_ns in net/net_namespace.h that is
available when the networking stack is not built at all move the
correct version of copy_net_ns from net_namespace.c into net_namespace.h
Leaving us with just 2 versions of copy_net_ns.  One version for when
we compile in network namespace suport and another stub for all other
occasions.
Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

include/net/net_namespace.h

@@ -126,16 +126,19 @@ struct net {
 /* Init's network namespace */
 extern struct net init_net;
 
-#ifdef CONFIG_NET
+#ifdef CONFIG_NET_NS
 extern struct net *copy_net_ns(unsigned long flags, struct net *net_ns);
 
-#else /* CONFIG_NET */
-static inline struct net *copy_net_ns(unsigned long flags, struct net *net_ns)
+#else /* CONFIG_NET_NS */
+#include <linux/sched.h>
+#include <linux/nsproxy.h>
+static inline struct net *copy_net_ns(unsigned long flags, struct net *old_net)
 {
-	/* There is nothing to copy so this is a noop */
-	return net_ns;
+	if (flags & CLONE_NEWNET)
+		return ERR_PTR(-EINVAL);
+	return old_net;
 }
-#endif /* CONFIG_NET */
+#endif /* CONFIG_NET_NS */
 
 
 extern struct list_head net_namespace_list;

net/core/net_namespace.c

@@ -347,13 +347,6 @@ struct net *get_net_ns_by_fd(int fd)
 }
 
 #else
-struct net *copy_net_ns(unsigned long flags, struct net *old_net)
-{
-	if (flags & CLONE_NEWNET)
-		return ERR_PTR(-EINVAL);
-	return old_net;
-}
-
 struct net *get_net_ns_by_fd(int fd)
 {
 	return ERR_PTR(-EINVAL);