iommufd: Prevent ALIGN() overflow

In the Linux kernel, the following vulnerability has been resolved:

iommufd: Prevent ALIGN() overflow

When allocating IOVA the candidate range gets aligned to the target alignment. If the range is close to ULONG_MAX then the ALIGN() can wrap resulting in a corrupted iova.

Open code the ALIGN() using get_add_overflow() to prevent this. This simplifies the checks as we don't need to check for length earlier either.

Consolidate the two copies of this code under a single helper.

This bug would allow userspace to create a mapping that overlaps with some other mapping or a reserved range.