This eliminated most of the remaining users of btfixup.

There are some complications because of the special cases we
have for sun4d, leon, and some flavors of viking.

It was found that there are no cases where a flush_page_for_dma
method was not hooked up to something, so the "noflush" iommu
methods were removed.

Add some documentation to the viking_sun4d_smp_ops to describe exactly
the hardware bug which causes us to need special TLB flushing on
sun4d.

Signed-off-by: David S. Miller <davem@davemloft.net>