get_net_ns_by_fd() oopses if proc_ns_fget() returns an error

BTW, looking through the code related to struct net lifetime rules has
caught something else:

struct net *get_net_ns_by_fd(int fd)
{
        ...
        file = proc_ns_fget(fd);
        if (!file)
                goto out;

        ei = PROC_I(file->f_dentry->d_inode);

while in proc_ns_fget() we have two return ERR_PTR(...) and not a single
path that would return NULL.  The other caller of proc_ns_fget() treats
ERR_PTR() correctly...
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/net_namespace.c

@@ -310,19 +310,17 @@ struct net *get_net_ns_by_fd(int fd)
 	struct file *file;
 	struct net *net;
 
-	net = ERR_PTR(-EINVAL);
 	file = proc_ns_fget(fd);
-	if (!file)
-		goto out;
+	if (IS_ERR(file))
+		return ERR_CAST(file);
 
 	ei = PROC_I(file->f_dentry->d_inode);
-	if (ei->ns_ops != &netns_operations)
-		goto out;
+	if (ei->ns_ops == &netns_operations)
+		net = get_net(ei->ns);
+	else
+		net = ERR_PTR(-EINVAL);
 
-	net = get_net(ei->ns);
-out:
-	if (file)
-		fput(file);
+	fput(file);
 	return net;
 }