DDF: compare_super_ddf: fix sequence number check

DDF: compare_super_ddf: fix sequence number check

The sequence number check in compare_super_ddf was broken,
anchor sequence number is always -1.

With this patch, mdadm will refuse to add a disk with non-matching
sequence number.

This fixes Francis Moreau's problem reported with subject
"mdadm 3.3 fails to kick out non fresh disk".

FIXME: More work is needed here. Currently mdadm won't even add the
disk to the container, that's wrong. It should be added as a spare.

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