By Karen Brettell and Tom Wilson NEW YORK/LONDON (Reuters) – Bitcoin gained on the day on Monday as buyers stepped in to buy the cryptocurrency, after earlier tumbling to its lowest level in six months on fears of a Russian attack on Ukraine and before this week’s Federal Reserve meeting. The largest cryptocurrency was last up 2.5% at $37,250 after earlier dropping to $32,951, its lowest price since July 23. That took losses from its all-time high of $69,000 hit in November past 50%. Bitcoin is now at a critical juncture where analysts say that further selling could reverse its long-term bull …