drbd: Fix for a race between IO and a detach operation [Bugz 262]

In D_DISKLESS we do not hand out any new references to ldev (local_cnt)
therefore waiting until all previously handed out refereces got returned
is sufficient before actually freeing mdev->ldev.

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