Incremental: remove old devices when assembling in container.

Incremental: remove old devices when assembling in container.

When assembling a native array we just give all devices to the kernel
and leave it to discard the 'old' ones (based on sequence/event
number).

For external/container arrays, mdadm needs to do that.

So in assemble_container_content, get list of current devices in
array and discard any that aren't in the 'content' given.
They must have been rejected by metadata manager.

If we cannot discard old devices the array must already be active, so
just leave it alone, but with a message.

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