ARM: dmabounce: move decision for bouncing into __dma_map_page()

Move the decision whether to bounce into __dma_map_page(), before
the check for high pages.  This avoids triggering the high page
check for devices which aren't using dmabounce.  Fix the unmap path
to cope too.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>