swiotlb: fix inversed DMA_ATTR_NO_WARN test

The result was printing the warning only when we were explicitly asked
not to.

Cc: stable@vger.kernel.org
Fixes: 0176adb0

 "swiotlb: refactor
 coherent buffer allocation"
Signed-off-by: Michel Dänzer <michel.daenzer@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com&gt;.>
Signed-off-by: Christoph Hellwig <hch@lst.de>