cleanup: quite gcc warn

gcc can't see dev_get_primary_dev  returns only 0,1,2
so ensure 'name' is always defined in valid path.

lib/filters/filter-mpath.c

@@ -158,6 +158,7 @@ static int dev_is_mpath(struct dev_filter *f, struct device *dev)
 		return 0;
 
 	switch (dev_get_primary_dev(dt, dev, &primary_dev)) {
+		default:
 		case 0:
 			/* Error. */
 			log_error("Failed to get primary device for %d:%d.", major, minor);