By Saqib Iqbal Ahmed NEW YORK (Reuters) -Dealers squaring their books ahead of an options expiration that is set to be the largest on record for S&P 500-linked derivatives may be helping to tamp down swings in U.S. stocks, market participants said. Some $5 trillion in U.S. stock options are set to expire on Friday, 80% in S&P 500-linked contracts – the largest such expiration in at least 20 years – according to Asym500 MRA Institutional, a unit of derivatives strategy and execution firm Macro Risk Advisors. While such events can exacerbate volatility, strategists say market participants’ behav…