* update_partition() split into add_partition() and delete_partition().
	* all updating of ->part[] is switched to these two (including initial
filling/final cleaning).
	* per-partition devices are allocated on-demand and never reused.
We allocate struct device in add_partition() and put reference to it into
hd_struct.  ->release() for that struct device frees it.  delete_partition()
removes reference from hd_struct and does put_device() on it.  Basically,
we get rid of problems with reused struct device by never reusing them...
	At that point devices for partitions are nice and sane.