WALTHAM, Mass. — The Celtics are angry. At the Knicks. At themselves. But mainly at the Knicks. While they were being blown out at home Friday, the Celtics felt the Knicks were showboating, rubbing Boston's collective nose into the same floor they wiped with the Celtics team. “Yeah, we did. We all did. That's what made us so mad, so angry,” guard Avery Bradley said before the Celtics' film session yesterday. “We just got to go out there and play the right way. We've got to not let that happen again. After the game, that was definitely something [discussed]. Somebody said it right after we got in the locker room: 'Oh, they're showboating on us, they're dunking.'