net: fix get_net_ns_by_fd for !CONFIG_NET_NS

After merging the final tree, today's linux-next build (powerpc
ppc44x_defconfig) failed like this:

net/built-in.o: In function `get_net_ns_by_fd':
(.text+0x11976): undefined reference to `netns_operations'
net/built-in.o: In function `get_net_ns_by_fd':
(.text+0x1197a): undefined reference to `netns_operations'

netns_operations is only available if CONFIG_NET_NS is set ...

Caused by commit f0630529 ("net: Allow setting the network namespace
by fd").
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

net/core/net_namespace.c

@@ -316,6 +316,28 @@ void __put_net(struct net *net)
 }
 EXPORT_SYMBOL_GPL(__put_net);
 
+struct net *get_net_ns_by_fd(int fd)
+{
+	struct proc_inode *ei;
+	struct file *file;
+	struct net *net;
+
+	net = ERR_PTR(-EINVAL);
+	file = proc_ns_fget(fd);
+	if (!file)
+		goto out;
+
+	ei = PROC_I(file->f_dentry->d_inode);
+	if (ei->ns_ops != &netns_operations)
+		goto out;
+
+	net = get_net(ei->ns);
+out:
+	if (file)
+		fput(file);
+	return net;
+}
+
 #else
 struct net *copy_net_ns(unsigned long flags, struct net *old_net)
 {
@@ -323,6 +345,11 @@ struct net *copy_net_ns(unsigned long flags, struct net *old_net)
 		return ERR_PTR(-EINVAL);
 	return old_net;
 }
+
+struct net *get_net_ns_by_fd(int fd)
+{
+	return ERR_PTR(-EINVAL);
+}
 #endif
 
 struct net *get_net_ns_by_pid(pid_t pid)
@@ -345,28 +372,6 @@ struct net *get_net_ns_by_pid(pid_t pid)
 }
 EXPORT_SYMBOL_GPL(get_net_ns_by_pid);
 
-struct net *get_net_ns_by_fd(int fd)
-{
-	struct proc_inode *ei;
-	struct file *file;
-	struct net *net;
-
-	net = ERR_PTR(-EINVAL);
-	file = proc_ns_fget(fd);
-	if (!file)
-		goto out;
-
-	ei = PROC_I(file->f_dentry->d_inode);
-	if (ei->ns_ops != &netns_operations)
-		goto out;
-
-	net = get_net(ei->ns);
-out:
-	if (file)
-		fput(file);
-	return net;
-}
-
 static int __init net_ns_init(void)
 {
 	struct net_generic *ng;