virtio: console: Disable lseek(2) for port file operations

The ports are char devices; do not have seeking capabilities.  Calling
nonseekable_open() from the fops_open() call and setting the llseek fops
pointer to no_llseek ensures an lseek() call from userspace returns
-ESPIPE.
Signed-off-by: Amit Shah <amit.shah@redhat.com>
CC: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

drivers/char/virtio_console.c

@@ -809,6 +809,8 @@ static int port_fops_open(struct inode *inode, struct file *filp)
 	reclaim_consumed_buffers(port);
 	spin_unlock_irq(&port->outvq_lock);
 
+	nonseekable_open(inode, filp);
+
 	/* Notify host of port being opened */
 	send_control_msg(filp->private_data, VIRTIO_CONSOLE_PORT_OPEN, 1);
 
@@ -840,6 +842,7 @@ static const struct file_operations port_fops = {
 	.poll  = port_fops_poll,
 	.release = port_fops_release,
 	.fasync = port_fops_fasync,
+	.llseek = no_llseek,
 };
 
 /*