The mips partition table in the volume header uses 512-byte logical units,

The mips partition table in the volume header uses 512-byte logical units,
not sectors; don't multiply by the sector size to get the proper disk offsets.

This will let install.iso be built with the OpenBSD label at the expected
location, instead of within the ffs filesystem; we had been lucky enough the
area being overwritten was not in use so far.