Fix gcc missing case warnings

Gcc ASSERT() missing cases are impossible

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/vdev_label.c

@@ -565,6 +565,8 @@ vdev_inuse(vdev_t *vd, uint64_t crtxg, vdev_labeltype_t reason,
 
 		case VDEV_LABEL_SPARE:
 			return (spa_has_spare(spa, device_guid));
+		default:
+			break;
 		}
 	}