Change how sudden-degraded devices should appear.
We don't record failure, we record that the device isn't there.
Signed-off-by: NeilBrown <neilb@suse.de>
If we assemble a newly-degraded array, the missing devices must be marked
as 'failed' so we don't expect them in future.
Signed-off-by: NeilBrown <neilb@suse.de>