bits_wanted is expanded to bytes using the wrong shift value
(when iovp_shift != PAGE_SHIFT), resulting in an explosion of
used iommu resources.

This potentially results in mistakenly running out of DMA mapping
resources when the system is under *heavy* i/o load.
Signed-off-by: Nigel Croxon <nigel.croxon@hp.com>
Signed-off by: Alex Williamson <alex.williamson@hp.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>