drbd: take error path in drbd_adm_down if interrupted by signal

drbd_adm_down() does adm_detach(), which can fail with various error
codes, or be interrupted by a signal.

The interrupted by signal case was not properly handled,
leading to
	block drbd0: ASSERT( mdev->state.disk == D_DISKLESS &&
	                     mdev->state.conn == C_STANDALONE ) in drbd/drbd_worker.c
and further to destroying objects while still in use, and resulting crashes.

Detect the interruption, and take the error path out.
Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>

drivers/block/drbd/drbd_nl.c

@@ -3188,7 +3188,7 @@ int drbd_adm_down(struct sk_buff *skb, struct genl_info *info)
 	/* detach */
 	idr_for_each_entry(&adm_ctx.tconn->volumes, mdev, i) {
 		retcode = adm_detach(mdev, 0);
-		if (retcode < SS_SUCCESS) {
+		if (retcode < SS_SUCCESS || retcode > NO_ERROR) {
 			drbd_msg_put_info("failed to detach");
 			goto out;
 		}