Avoid removing an item in iommu_gas_free_region only to reinsert it, by avoiding removing an entry that is either first_place or last_place.
This is the only use I find of either first_place or last_place data, so it could be written using prev and next instead. This version saves time in calls to iommu_gas_free_region, while removing those fields would save space. I could go either way.