iommufd: Don't overflow during division for dirty tracking

In the Linux kernel, the following vulnerability has been resolved: iommufd: Don't overflow during division for dirty tracking If pgshift is 63 then BITS_PER_TYPE(*bitmap->bitmap) * pgsize will overflow to 0 and this triggers divide by 0. In this case the index should just be 0, so reorganize things to divide by shift and avoid hitting any overflows.