drbd: fix DRBD_VOLUME_MAX 65535 -> 65534

drbd: fix DRBD_VOLUME_MAX 65535 -> 65534

The protocol uses -1 as a reserved value for
'no specific volume', and since the protocol field
is a 16 bit unsigned value, -1 is converted to
65535. Therefore, limit the range of valid volume
numbers to [0, 65534].

Signed-off-by: Robert Altnoeder <robert.altnoeder@linbit.com>
Signed-off-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Reviewed-by: Joel Colledge <joel.colledge@linbit.com>
Link: https://lore.kernel.org/r/20230113123538.144276-3-christoph.boehmwalder@linbit.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>