dma-mapping: move various slow path functions out of line

There is no need to have all setup and coherent allocation / freeing
routines inline.  Move them out of line to keep the implemeation
nicely encapsulated and save some kernel text size.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Tested-by: Tony Luck <tony.luck@intel.com>