DDF: getinfo_super_ddf_bvd: fix raid_disk calculation

DDF: getinfo_super_ddf_bvd: fix raid_disk calculation

The return value of disk.raid_disk may be wrong.
The old code was using raiddisk, which is only valid with auto
layout. This leads to errors when arrays are created with
specified disks and mdmon is already running, like this:

mdadm -CR /dev/md/container -n5 $d1 $d2 $d3 $d4 $d5
mdadm -CR /dev/md/r5 -n5 -l5 /dev/md/container -z 5000
mdadm -CR /dev/md/r1 -n2 -l1 $d1 $d2
  => resulting array will use wrong disks

This patch fixes that.

Signed-off-by: NeilBrown <neilb@suse.de>