iommu/iova: Extend cached node lookup condition

When searching for a free IOVA range, we optimise the tree traversal
by starting from the cached32_node, instead of the last node, when
limit_pfn is equal to dma_32bit_pfn. However, if limit_pfn happens to
be smaller, then we'll go ahead and start from the top even though
dma_32bit_pfn is still a more suitable upper bound. Since this is
clearly a silly thing to do, adjust the lookup condition appropriately.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>