struct gendisk and partition parsers divorced; all these parsers (IBM style,
disklabel, etc.) just fill the structure they get from check_partitions().

Actual setting the things up (filling hd_struct arrays, telling RAID that
we had found partitions worth a look, etc.) is taken into check_partitions()
and done only when we are done with parsing.  Parsers don't know (or care)
what majors/minors they are dealing with; that knowledge also went to
check_partitions().