Account for the offset of the page run when allocating the
dmar_map_entry. Non-zero offset both increases the required mapping
size, which is handled in dmar_bus_dmamap_load_something1(), and makes
it possible that allocated range crosses boundary, which needs a check
in dmar_gas_match_one().

Reported and tested by: jimharris
Sponsored by: The FreeBSD Foundation
MFC after: 1 week