drbd: adjust drbd_limits license header

See also commit 93c68cc46a07 ("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 <[e-mail]>
Reviewed-by: Joel Colledge <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

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.