net/virtio: Drop useless n->primary_dev not null checks

net/virtio: Drop useless n->primary_dev not null checks

virtio_net_handle_migration_primary() returns early when it can't
ensure n->primary_dev is non-null.  Checking it again right after that
early return is redundant.  Drop.

If n->primary_dev is null on entering failover_replug_primary(), @pdev
will become null, and pdev->partially_hotplugged will crash.  Checking
n->primary_dev later is useless.  It can't actually be null, because
its caller virtio_net_handle_migration_primary() ensures it isn't.
Drop the useless check.

Cc: Jens Freimann <jfreimann@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20191130194240.10517-2-armbru@redhat.com>
Reviewed-by: Jens Freimann <jfreimann@redhat.com>