drbd: use blk_set_stacking_limits()

...instead directly assigning to q->limits.discard_zeroes_data

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