* Iraqi army challenged by casualties, desertion * Tribal fighters fear they are outmanned * ****** supplies, air strikes insufficient, fighters say By Ahmed Rasheed, Saif Hameed and Ned Parker BAGHDAD, Oct 21 (Reuters) - In recent weeks, the world has watched the battle to save Syria's border town of Kobani from Islamic State. But the radical jihadists have for longer been engulfing a**ther strategically more vital target - Iraq's western Anbar province and its road to Baghdad. ...