RDMA/restrack: Change SPDX tag to properly reflect license

Resource tracking is supposed to be dual licensed: GPL-2.0 and
OpenIB, but the SPDX tag was not compliant to it. Update the tag to
properly reflect license.

Fixes: 02d8883f

 ("RDMA/restrack: Add general infrastructure to track RDMA resources")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>