analyse_change: fix calculation of after.data_disks and ->delta_disks.

analyse_change: fix calculation of after.data_disks and ->delta_disks.

When changing level when a new number of raid disks was explicitly
specified, we much make sure that the change implied by the
change in level is properly incorporated into the final result.

So explicitly track the change in number of parity disks
(delta_parity) and use it together with delta_disks to determine
final data_disks.
Also set info->delta_disks so other code doesn't need to mirror
this analysis.

And add some errors in cases where a new number of disks was
requested but is not currently supported

Reported-by: Adam Kwolek <adam.kwolek@intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>