x86, pci: iommu fix potential overflow in alloc_iommu()

It is possible that alloc_iommu()'s boundary_size overflows as
dma_get_seg_boundary can return 0xffffffff.  In that case, further usage of
boundary_size triggers a BUG_ON() in the iommu code.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>