This very same question has puzzled me for some time. You're right to note that its an omnipresent trope in Russian Civil War movies to show soldiers, especially the Whites, advancing slowly forward into machine-gun fire in closed ranks, bayonets fixed. Not only does it appear in 2008's Admiral, but also in older movies, including 1934's Chapeav, 1965's Doctor Zhivago, when the Red partisans mow down attacking columns of cadets, and in 1967's The Red and the White, which shows both sides fighting like that, or even in this Russian film I have never seen before, but which Youtube helpfully suggested

So it is common in film, but did it happen in real life? You're right to note that it looks a little crazy, and unfortunately I have to preface my answer by saying that this is a little hard to answer, simply because so few English-language sources I've seen talk about actual battle tactics in the Russian Civil War, or even the Eastern Front of the First World War. However, I feel confident saying that yes, such tactics did occur, although not all the time.

First of all, there is totally evidence that frontal attacks like this occurred. For example, during the Red Army's counter-offensive against Admiral Kolchak in the autumn of 1919, a Red officer recorded one such attack near Ufa, where a White shock battalion, consisting entirely of officers who had received the prestigious Order of St. George, launched a last-ditch assault to keep the town from falling. They White officers "emerged out of the dawn and advanced towards the Red machine guns, 'with fixed bayonets, silent, holding their fire'. With 'skull and crossbones insignia mounted on their caps, sleeves and epaulettes, they made a terrifying impression."

Another example from a primary sources comes from the memoirs of A. Z. Ovchinnikov, a teenager from Ufa who fought on both sides of the Russian Civil War, first with the anti-Bolshevik Czech Legion, then in Kolchak's White Army, and then as a Red partisan fighting against Kolchak. He describes his first action with the Czechs, attacking a force of Red partisans in 1918. The tactics he describes are typical WWI or pre-WWI infantry tactics: platoons go forward in a skirmish line, laying down to fire at the enemy, and then moving while others give them covering fire.

So yes, it did happen, although I suspect films like to portray it as a way of showing the Whites as overly tradition-bound, or something. But why did it happen in real life? To be continued!

Jonathan D. Smele, Civil War in Siberia: The anti-Bolshevik government of Admiral Kolchak, 1918-1920

A. Z. Ovchinnikov, Memoirs of the Red Partisan Movement in the Russian Far East