bit_map_string_get
int bit_map_string_get(struct bit_map *t, int len, int align);
ioptex = bit_map_string_get(&iommu->usemap, npages, pfn);
ioptex = bit_map_string_get(&iommu->usemap, len >> PAGE_SHIFT,
offset = bit_map_string_get(&srmmu_nocache_map,