[NET]: cleanup sock_from_file()

I believe dead code from sock_from_file() can be cleaned up.

All sockets are now built using sock_attach_fd(), that puts the 'sock' pointer 
into file->private_data and &socket_file_ops into file->f_op

I could not find a place where file->private_data could be set to NULL, 
keeping opened the file.

So to get 'sock' from a 'file' pointer, either :

- This is a socket file (f_op == &socket_file_ops), and we can directly get 
'sock' from private_data.
- This is not a socket, we return -ENOTSOCK and dont even try to find a socket 
via dentry/inode :)
Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/socket.c

@@ -407,24 +407,11 @@ int sock_map_fd(struct socket *sock)
 
 static struct socket *sock_from_file(struct file *file, int *err)
 {
-	struct inode *inode;
-	struct socket *sock;
-
 	if (file->f_op == &socket_file_ops)
 		return file->private_data;	/* set in sock_map_fd */
 
-	inode = file->f_path.dentry->d_inode;
-	if (!S_ISSOCK(inode->i_mode)) {
-		*err = -ENOTSOCK;
-		return NULL;
-	}
-
-	sock = SOCKET_I(inode);
-	if (sock->file != file) {
-		printk(KERN_ERR "socki_lookup: socket file changed!\n");
-		sock->file = file;
-	}
-	return sock;
+	*err = -ENOTSOCK;
+	return NULL;
 }
 
 /**