[PATCH] swiotlb: support syncing sub-ranges of mappings

This patch implements swiotlb_sync_single_range_for_{cpu,device}. This
is intended to support an x86_64 implementation of
dma_sync_single_range_for_{cpu,device}.

Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>