torah study - How could Rebbi Zeira forcibly forget his learning?

In Bava Metzia 85a it says that

ר' זירא כי סליק לארעא דישראל יתיב מאה תעניתא דלשתכח גמרא בבלאה מיניה

Rebbi Zeira, when he left to the land of Israel, fasted 100 times to cause himself to forget the Babylonian talmud.

How was this allowed given that the mishnah in avos 3:8 says that anyone who forcibly forgets his torah learning is culpable for his life?