drbd: don't pretend that barrier_nr == 0 was special

We used to have a barrier implementation where barrier_nr 0 was
reserved. That is long gone. Just use the full sequence space.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>