Fix Intel IOMMU write-buffer flushing

This is the cause of the DMA faults and disk corruption that people have
been seeing. Some chipsets neglect to report the RWBF "capability" --
the flag which says that we need to flush the chipset write-buffer when
changing the DMA page tables, to ensure that the change is visible to
the IOMMU.

Override that bit on the affected chipsets, and everything is happy
again.

Thanks to Chris and Bhavesh and others for helping to debug.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>
Tested-by: Chris Wright <chrisw@sous-sol.org>
Reviewed-by: Bhavesh Davda <bhavesh@vmware.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>