kup: get_data_format() takes the data, not $fmt

get_data_format() takes the data, not $fmt, which will be undef at
this point anyway.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/kup b/kup
index 5cadede..8b8436f 100755
--- a/kup
+++ b/kup
@@ -248,7 +248,7 @@
 	    }
 
 	    if (!defined($fmt)) {
-		$fmt = get_data_format($fmt);
+		$fmt = get_data_format($blk);
 		print "${cmd} ${file} ${fmt}\n";
 	    }