drbd: make limits unsigned

These are almost always used as unsigned integers, so mark them as such.
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Reviewed-by: Joel Colledge <joel.colledge@linbit.com>
Link: https://lore.kernel.org/r/20230113123538.144276-4-christoph.boehmwalder@linbit.comSigned-off-by: Jens Axboe <axboe@kernel.dk>