This triggers for b44's 1GB DMA workaround which tries to map
first and then bounces.

The 32bit heuristic is reasonable because the IOMMU doesn't attempt
to handle < 32bit masks anyways.
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>