one liner DMA mapping fix for 4.20

From: Christoph Hellwig
Date: Sat Dec 22 2018 - 14:03:28 EST

Hi Linus,

I'm sending this directly to you given how late we are. This is a trivial
one liner fixing a regression I introduces in this merge merge, and which
has only been noticed by code inspection, but which is pretty obvious when
looking at the original commit. It would be great if it could make 4.20.