Obviously here in Smaug’s lair, Tolkien means that the benches are toppled.  I am intrigues to learn that the word can also mean “rotate” as a wheel, to fall to grief or ruin, or to disorder a stomach.

• 13.046 and benches were lying there overturned,

“overturn, v.” OED Online. Oxford University Press, June 2015. Web. 14 July 2015.