So far as I know, the shortest thing for which copyright has been successfully defended was a one-liner by Ashleigh Brilliant (which, obviously, I can’t reproduce here legally). But that was both longer than the Tolkien quote, and a “complete work” in its own right (as opposed to a fraction of a poem, or a minuscule fraction of a book), so I would imagine that “Not all who wander are lost” would not be copyrightable.
That’s what made me ask the question. I thought the shirt was a neat concept but I didn’t like the way it was carried out so I searched for other designs. When I didn’t find any it made me ponder whether your shirt designer was the only one who had rights to use the phrase.