drbd: adjust drbd_limits license header

See also commit 93c68cc4 ("drbd: use consistent license"). We only
want to license drbd under GPL-2.0, so use the corresponding SPDX header
consistently.
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Reviewed-by: Joel Colledge <joel.colledge@linbit.com>
Link: https://lore.kernel.org/r/20230113123538.144276-2-christoph.boehmwalder@linbit.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/drbd_limits.h

-/* SPDX-License-Identifier: GPL-2.0 */
+/* SPDX-License-Identifier: GPL-2.0-only */
 /*
   drbd_limits.h
   This file is part of DRBD by Philipp Reisner and Lars Ellenberg.