xfrm: handle NETDEV_UNREGISTER for xfrm device

This patch to handle the asynchronous unregister
device event so the device IPsec offload resources
could be cleanly released.

Fixes: e4db5b61c572 ("xfrm: policy: remove pcpu policy cache")
Signed-off-by: Raed Salem <[e-mail]>
Reviewed-by: Boris Pismenny <[e-mail]>
Reviewed-by: Saeed Mahameed <[e-mail]>
Signed-off-by: Steffen Klassert <[e-mail]>

net/xfrm/xfrm_device.c

···
 		return xfrm_dev_feat_change(dev);
 
 	case NETDEV_DOWN:
+	case NETDEV_UNREGISTER:
 		return xfrm_dev_down(dev);
 	}
 	return NOTIFY_DONE;