drbd: Fix postponed requests

* Postponed requests should not set or clear out-of-sync marks
* When a request gets postponed we need to drop its reference
  mdev->local_cnt (put_ldev()).

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