This fixes a bug in the PPC64 iommu vmerge code which results in the
potential for iommu_unmap_sg to go off unmapping more than it should.

This was found on a test system which resulted in PCI bus errors due to
PCI memory being unmapped while DMAs were still in progress.
Signed-off-by: Brian King <brking@us.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>