DDF: Implement store_super_ddf

DDF: Implement store_super_ddf

This patch implements the previously unsupported case where
store_super_ddf is called with a non-empty superblock.

For DDF, writing meta data to just one disk makes no sense.
We would run the risk of writing inconsistent meta data
to the devices. So just call __write_init_super_ddf and
write to all devices, including the one passed by the caller.

This patch assumes that the device to store the superblock on
has already been added to the DDF structure. Otherwise, an
error message will be emitted.

Signed-off-by: Martin Wilck <mwilck@arcor.de>
Signed-off-by: NeilBrown <neilb@suse.de>