use generic DMA mapping code in powerpc V4

[Christoph Hellwig]

Thanks for bisecting.  I've spent some time going over the conversion
but can't really pinpoint it.  I have three little patches that switch
parts of the code to the generic version.  This is on top of the
last good commmit (977706f9755d2d697aa6f45b4f9f0e07516efeda).

Can you check with whіch one things stop working?


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