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.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>