Update the scatterlist logic so that PIO options are also disabled
when an IOMMU may have coalesced pages during dma_map_sg() ... it's
not just HIGHMEM that can make trouble supporting both PIO and DMA
based host controller drivers.

There also seems to be a cross-arch issue here, with 64bit powerpc
not using an IOMMU define ... and its IOMMU_VMERGE config can always
be overridden on the kernel command line.  So this is better, but
still imperfect.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>