9p/xen: Add cleanup path in p9_trans_xen_init

commit 80a316ff16276b36d0392a8f8b2f63259857ae98 upstream.

If xenbus_register_frontend() fails in p9_trans_xen_init,
we should call v9fs_unregister_trans() to do cleanup.

Link: http://lkml.kernel.org/r/20190430143933.19368-1-yuehaibing@huawei.com
Cc: stable@vger.kernel.org
Fixes: 868eb122 ("xen/9pfs: introduce Xen 9pfs transport driver")
Signed-off-by: NYueHaibing <yuehaibing@huawei.com>
Signed-off-by: NDominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

net/9p/trans_xen.c

@@ -530,13 +530,19 @@ static struct xenbus_driver xen_9pfs_front_driver = {
 
 static int p9_trans_xen_init(void)
 {
+	int rc;
+
 	if (!xen_domain())
 		return -ENODEV;
 
 	pr_info("Initialising Xen transport for 9pfs\n");
 
 	v9fs_register_trans(&p9_xen_trans);
-	return xenbus_register_frontend(&xen_9pfs_front_driver);
+	rc = xenbus_register_frontend(&xen_9pfs_front_driver);
+	if (rc)
+		v9fs_unregister_trans(&p9_xen_trans);
+
+	return rc;
 }
 module_init(p9_trans_xen_init);