drbd: check on CONFIG_LBDAF, not LBD

It is called LBDAF since 2.6.31.

impact:
without this change, on 32bit,
DRBD would wrongly claim to only support 2TiB devices.

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