imsm: getinfo_super_imsm_volume() doesn't fill all disk information

imsm: getinfo_super_imsm_volume() doesn't fill all disk information

getinfo_super_imsm_volume doesn't correctly set info.disk fields
because it doesn't know which disk to set them from.
It should be the last disk passed to add_to_super.

So add a field 'current_disk' to record this disk in add_to_super, and
use it in getinfo_super.

This allows us to remove a hack in Create.c

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