[Lab-2] Finish Lab-2

[Description]
- Finish unlock_mutex()
- Add Comment

Signed-off-by: C.W. Hsiao <cwhsiao.lionel@gmail.com>
[Lab-2] Change magic number "2" to MAX_THREAD

Signed-off-by: C.W. Hsiao <cwhsiao.lionel@gmail.com>