9p/xen: Add cleanup path in p9_trans_xen_init

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: 868eb122739a ("xen/9pfs: introduce Xen 9pfs transport driver")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>