swiotlb: rename unmap_single to do_unmap_single

Previously, swiotlb_unmap_page and swiotlb_unmap_sg were
duplicating very similar code.  Refactor that code into a
new unmap_single and unmap_single use do_unmap_single.

Note that the swiotlb_unmap_sg code was previously doing
a complicated comparison to determine if an addresses needed
to be unmapped where a simple is_swiotlb_buffer() call
would have sufficed.

Signed-off-by: Becky Bruce <beckyb@kernel.crashing.org>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Cc: jeremy@goop.org
Cc: ian.campbell@citrix.com
LKML-Reference: <1239199761-22886-6-git-send-email-galak@kernel.crashing.org>
Signed-off-by: Ingo Molnar <mingo@elte.hu>