md/raid0: Ignore RAID0 layout if the second zone has only one device

The RAID0 layout is irrelevant if all members have the same size so the
array has only one zone. It is *also* irrelevant if the array has two
zones and the second zone has only one device, for example if the array
has two members of different sizes.

So in that case it makes sense to allow assembly even when the layout is
undefined, like what is done when the array has only one zone.

Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Pascal Hambourg <pascal@plouf.fr.eu.org>
Signed-off-by: Song Liu <song@kernel.org>