This was not noticed for many years. Affects operation if
md raid is used a backing device for DRBD.

CC: stable@kernel.org # v3.2+
Signed-off-by: NPhilipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: NLars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: NJens Axboe <axboe@fb.com>