6lowpan: ndisc: fix double read unlock

This patch removes a double unlock case to accessing neighbour private
data.
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Alexander Aring <aar@pengutronix.de>
Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/6lowpan/ndisc.c

@@ -135,8 +135,9 @@ static int lowpan_ndisc_opt_addr_space(const struct net_device *dev,
 			read_unlock_bh(&neigh->lock);
 			addr_space += __ndisc_opt_addr_space(IEEE802154_SHORT_ADDR_LEN, 0);
 			*ha = ha_buf;
-		}
+		} else {
 			read_unlock_bh(&neigh->lock);
+		}
 		break;
 	case NDISC_NEIGHBOUR_ADVERTISEMENT:
 	case NDISC_NEIGHBOUR_SOLICITATION: