dev/iommu: Include offset in maxaddr check.

If iommu_gas_match_one has to adjust for a boundary crossing, its
check against maxaddr includes 'offset' in its calculation, to ensure
that the allocated memory does not exceed the max address. However, if
there's no boundary crossing adjustment, then the maxaddr check
disregards 'offset'. Fix that.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D34978