How do you get turmeric out of white linen?

Hydrogen peroxide-based cleaners such as Oxiclean are my go-to for removing any type of stain; they work amazingly well and are also color-safe (although aren't ok for some synthetics, silk, and wool). For heavy stains, apply a bit directly to the spot just before washing, or soak overnight in a diluted bath of it.
If you want to use chlorine bleach instead, make sure you wash the fabric first, because the bleach reacts with any residual food to make a compound that will yellow over time.

https://food52.com/blog/15548-how-to-get-rid-of-turmeric-stains-on-your-dishes-linens-counters-and-fingers

I haven't tried it in white linen, but in my experience, turmeric stains usually disappear, or at least fade with sun + acidity. Try a wash in cool water and hang in the sun with a spritz of vinegar. I've had this work on towels where bleach didn't get out the stain.