pre-dump: handle only seized tasks

In a previous patch, we skipped zombies, but our pre-dump doesn't
support stopped tasks (Why does it not support them?), so let's
handle tasks which have parasite_ctl.

Cc: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Andrew Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/cr-dump.c

@@ -1432,7 +1432,7 @@ static int cr_pre_dump_finish(int ret)
 		struct parasite_ctl *ctl = dmpi(item)->parasite_ctl;
 		struct page_xfer xfer;
 
-		if (!task_alive(item))
+		if (!ctl)
 			continue;
 
 		pr_info("\tPre-dumping %d\n", ctl->pid.virt);