netfilter: xt_helper: Use sizeof(variable) instead of literal number

It's better to use sizeof(info->name)-1 as index to force set the string
tail instead of literal number '29'.
Signed-off-by: Gao Feng <fgao@ikuai8.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/xt_helper.c

@@ -65,7 +65,7 @@ static int helper_mt_check(const struct xt_mtchk_param *par)
 			par->family);
 		return ret;
 	}
-	info->name[29] = '\0';
+	info->name[sizeof(info->name) - 1] = '\0';
 	return 0;
 }