tree-wide: if get_block_device() returns zero devno, check for it in all cases

And add a comment for the existing cases where things aren't clear
already.

src/volatile-root/volatile-root.c

@@ -175,7 +175,7 @@ static int run(int argc, char *argv[]) {
         r = get_block_device_harder(path, &devt);
         if (r < 0)
                 return log_error_errno(r, "Failed to determine device major/minor of %s: %m", path);
-        else if (r > 0) {
+        else if (r > 0) { /* backed by block device */
                 _cleanup_free_ char *dn = NULL;
 
                 r = device_path_make_major_minor(S_IFBLK, devt, &dn);