get_gendisk() now takes dev_t (instead of kdev_t) and gets an additional
argument - int *part.  Set to 0 for non-partitioned, partition number
for partititoned.  Callers updated.  Yes, I hate passing return values
that way ;-/  We need that since old "minor(dev) - disk->first_minor"
doesn't work for stuff with non-trivial numbers (e.g. floppy) and
get_gendisk() really has to return both gendisk and partition number.
Fortunately, amount of callers of gendisk() is about to drop RSN big way...