sched_ext/selftests: Fix format specifier and buffer length in file_write_long()

Use %ld (not %lu) for signed long, and pass the actual string length
returned by sprintf() to write_text() instead of sizeof(buf).

Signed-off-by: Cheng-Yang Chou <yphbchou0911@gmail.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: EricccTaiwan

tools/testing/selftests/sched_ext/util.c

@@ -60,11 +60,11 @@ int file_write_long(const char *path, long val)
 	char buf[64];
 	int ret;
 
-	ret = sprintf(buf, "%lu", val);
+	ret = sprintf(buf, "%ld", val);
 	if (ret < 0)
 		return ret;
 
-	if (write_text(path, buf, sizeof(buf)) <= 0)
+	if (write_text(path, buf, ret) <= 0)
 		return -1;
 
 	return 0;