Since ->nr_real is always 1 now, we can remove that field completely.
Removed the last remnants of switch in disk_name() (it could be killed
a long time ago, I just forgot to remove the last two cases when md and i2o
got converted).  Collapsed several instances of
disk->part[minor - disk->first_minor] - in cases when we know that we deal
with disk->part[0].