iommu_gas: pass size to iommu_map without rounding

Let the caller to iommu_map pass the size parameter without rounding
it up to a multiple of page size. Let iommu_map round it up when
necessary, which is not all of the time, so that in some cases less
space is reserved.

Reviewed by: alc, kib (previous version)
Tested by: pho, br
Discussed with: andrew
MFC after: 3 weeks
Differential Revision: https://reviews.freebsd.org/D35424