syscalls/chmod02: Fix test file creation + cleanup

1. Cleanup the code, remove useless comments.

2. The code to create the file is broken and overly complicated,
   replace it with SAFE_FILE_PRINTF().

Reported-by: jungsoo.son <jungsoo.son@lge.com>
Signed-off-by: Cyril Hrubis <chrubis@suse.cz>