This narration which you quoted (Hazrat Mahdi's father's name is Abdullah) is weak and not authentic even among Sunni scholars. The authentic Hadeeth stated : Al-Mahdi's name is like my name, and his Kunyah is like my Kuniyah). Liars added (His father's name is like my father's name.
Imam Al-Mahdi (AS) was born in Samarra in 266 Hijri. The birth of Imam Al-Mahdi (AS) has been mentioned by many Sunni leading scholars in their well known books.