Fix alignment of 'last' in autofill.

'last' is the sector number of the last usable sector. Sector
numbers start with 0. As such, 'last' is always 1 less than
the count of sectors and aligning 'last' down as-is means that
the number of free sectors is pessimized by 'alignment - 1' if
the number of usable sectors was already a multiple of the
alignment. Consequently, gpart(8) failed to create a partition
when the alignment and size were such that it would extend to
the end of the disk.