arm32: The cache may not be properly cleaned/invalidated

Arm provides multiple helpers to clean & invalidate the cache

for a given region. This is, for instance, used when allocating

guest memory to ensure any writes (such as the ones during scrubbing)

have reached memory before handing over the page to a guest.

Unfortunately, the arithmetics in the helpers can overflow and would

then result to skip the cache cleaning/invalidation. Therefore there

is no guarantee when all the writes will reach the memory.