dma-direct: provide generic support for uncached kernel segments

A few architectures support uncached kernel segments.  In that case we get
an uncached mapping for a given physica address by using an offset in the
uncached segement.  Implement support for this scheme in the generic
dma-direct code instead of duplicating it in arch hooks.

Signed-off-by: Christoph Hellwig <hch@lst.de>