drbd: Consider that read requests could be NEG_ACKEDed

ap_in_flight only counts writes. NEG_ACKED is an action
on a request that might be called for reads and writes.

This bug was there forever, but it becomes much more
relevant with the read balincing code.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_req.c

@@ -628,7 +628,8 @@ int __req_mod(struct drbd_request *req, enum drbd_req_event what,
 		/* assert something? */
 		if (req->rq_state & RQ_NET_PENDING) {
 			dec_ap_pending(mdev);
-			atomic_sub(req->i.size >> 9, &mdev->ap_in_flight);
+			if (req->rq_state & RQ_WRITE)
+				atomic_sub(req->i.size >> 9, &mdev->ap_in_flight);
 		}
 		req->rq_state &= ~(RQ_NET_OK|RQ_NET_PENDING);