replication: Reduce usage of s->hidden_disk and s->secondary_disk

replication: Reduce usage of s->hidden_disk and s->secondary_disk

In preparation for the next patch, initialize s->hidden_disk and
s->secondary_disk later and replace access to them with local variables
in the places where they aren't initialized yet.

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Message-Id: <1eb9dc179267207d9c7eccaeb30761758e32e9ab.1626619393.git.lukasstraub2@web.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>