- this one is interesting and will play in the next series as well;
   affected place is fs/block_dev.c::do_open().  We check if bdev is
   a partition (same way it is done in generic_make_request()) and
   if it is - open entire disk and put pointer to its bdev into a
   new field bdev->bd_contains.  Otherwise (non-partition) we
   set bdev->bd_contains to bdev.  Corresponding cleanup done in
   blkdev_put() (and failure path in do_open()) - when the last opener
   goes away we close bdev->bd_contains if bdev is a partition (i.e.
   not equal to its ->bd_contains) and set it to NULL.
   Immediate effect is that we can get from bdev of partition to
   bdev of disk when submitting a bio, but it also opens a way
   to handle partition-parsing in a sane way.  That will be done
   in the next series.