drbd: fix list corruption (recent regression)

The commit 288f422e
 drbd: Track all IO requests on the TL, not writes only
moved a list_add_tail(req, ) into a region where req
may have just been freed due to conflict detection.

Fix this by adding a proper cleanup section for that code path.

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