fix x86 swiotlb regression

[Christoph Hellwig]

Hi all,

this patch fixes a regression in the x86 swiotlb conversion.  This mostly
happend because swiotlb_dma_support does the wrong thing (and did so for
a long time) and we switched x86 to use it.

There are a few others users of swiotlb_dma_supported that also look
rather broken, but I'll take care of those for the next merge window.