new helpers - get_capacity(gendisk)/set_capacity(gendisk, sectors).
Drivers switched to these; that eliminates most of the accesses to
disk->part[]... in the drivers (and makes code more readable, while
we are at it).  That had caught several bugs when minor had been
used in place of minor>>minor_shift (acsi.c is especially nasty in
that respect; I don't know if it had ever been used with multiple
devices...)