ioat(4): Add support for 'fence' bit with DMA_FENCE flag

Some classes of IOAT hardware prefetch reads. DMA operations that
depend on the result of prior DMA operations must use the DMA_FENCE flag
to prevent stale reads.

(E.g., I've hit this personally on Broadwell-EP. The Broadwell-DE has a
different IOAT unit that is documented to not pipeline DMA operations.)

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