iommu_gas: Fix a recent regression with IOMMU_MF_CANSPLIT

As of 19bb5a7244ff, the IOMMU_MF_CANSPLIT case in iommu_gas_match_one()
must take into account the specified offset. Otherwise, the recently
changed end calculation in iommu_gas_match_insert() could produce an
end address that crosses the specified boundary by one page.

Reviewed by: dougm
MFC with: 19bb5a7244ff

(cherry picked from commit 7b39a9bc1df37502e8186593f3427b7ff0e4cc71)