DDF: fix removal of failed devices.

Commit c7079c84 arrange for DDF to forget about any device
that is failed and not still marked as part of any array.

However such devices could still be part of the container and this
removal and updating of 'pdnum' can result in multiple devices having
the same pdnum.  This in turn easily leads to confusion and
corruption.

So only discard pd entries for devices which are failed, not listed in
any virtual device, and for which we don't have a handle on the
device.

pd entries will not get removed until a new device is added after
the device has been removed from the container, either by
"mdadm --remove" or by assembling without the failed devices.

Reported-by: Albert Pauw <albert.pauw@gmail.com>
Analysed-by: Martin Wilck <mwilck@arcor.de>
Signed-off-by: NeilBrown <neilb@suse.de>