now that gendisks don't have shared stuff (we used to set blk_size[]
from ->sizes) we can start splitting them into per-disk ones.

Step 1:
  we introduce a new field - ->first_minor (to be merged with ->major
  into dev_t of entire disk once all gendisks are split).  All arrays
  are assumed to start at that minor (i.e.  gd->part[minor] got replaced
  with gd->part[minor - gd->first_minor], etc.).  get_gendisk() is
  taught to pick the right gendisk if there are several with the same
  major.