use SPDX-License-Identifier instead of a verbose license text

Link: https://lore.kernel.org/r/20210823023530.48-1-caihuoqing@baidu.comSigned-off-by: Cai Huoqing <caihuoqing@baidu.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>