implement O_NONBLOCK for /proc/xen/xenbus

This patch implements O_NONBLOCK for /proc/xen/xenbus.  It is a simple
matter of returning -EAGAIN instead of waiting on a queue.
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

drivers/xen/xenfs/xenbus.c

@@ -122,6 +122,9 @@ static ssize_t xenbus_file_read(struct file *filp,
 	mutex_lock(&u->reply_mutex);
 	while (list_empty(&u->read_buffers)) {
 		mutex_unlock(&u->reply_mutex);
+		if (filp->f_flags & O_NONBLOCK)
+			return -EAGAIN;
+
 		ret = wait_event_interruptible(u->read_waitq,
 					       !list_empty(&u->read_buffers));
 		if (ret)