drbd: properly call drbd_rs_cancel_all() in drbd_disconnected()

drbd_disconnected() is supposed to clear the resync lru cache,
by calling drbd_rs_cancel_all().

We must do so before we call drbd_flush_workqueue(), as at least the
callback w_restart_disk_io() may wait for resync progres, and would
otherwise deadlock.

drbd_finish_peer_reqs() may again populate that cache, which will
then potentially be stale after the next resync handshake and bitmap
exchange, we have to do it again after that.

A stale resync lru cache causes no harm but ugly messages like this:
 BAD! sector=196608s enr=6 rs_left=-256 rs_failed=0 count=256 cstate=SyncTarget
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_receiver.c

@@ -4526,6 +4526,10 @@ static int drbd_disconnected(struct drbd_conf *mdev)
 	   necessary to reclain net_ee in drbd_finish_peer_reqs(). */
 	drbd_flush_workqueue(mdev);
 
+	/* need to do it again, drbd_finish_peer_reqs() may have populated it
+	 * again via drbd_try_clear_on_disk_bm(). */
+	drbd_rs_cancel_all(mdev);
+
 	kfree(mdev->p_uuid);
 	mdev->p_uuid = NULL;