revert dma direct internals abuse

[Christoph Hellwig]

Hi Linus,

unfortunately it took less than a merge window for the:

/*
 * All the dma_direct_* declarations are here just for the indirect call bypass,
 * and must not be used directly drivers!
 */

warning in dma-mapping.h to be ignored.  This series reverts the offender
to keep our API clean.

Thomas: please talk me first about your needs and I'll be happy to figure out
a proper API for what you want to do.