swiotlb: add support for systems with highmem

Impact: extend code for highmem - existing users unaffected

On highmem systems, the original dma buffer might not
have a virtual mapping - we need to kmap it in to perform
the bounce.  Extract the code that does the actual
copy into a function that does the kmap if highmem
is enabled, and default to the normal swiotlb memcpy
if not.

[ ported by Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com> ]

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>