VMERGE enables you to handle a large request even with drivers with
small sg tables.
BTW, alpha IOMMU can't handle VMERGE. But the IOMMU has the code to
handle VMERGE so one-line patch can fix the IOMMU.
As I said before, can we leave this to Jens, keeping or removing
VMERGE? Seems that I see the same arguments again and again.