files: Make possible task helpers to use shared_fdt_prepare()

Next patches will create transport sockets in task helpers.
As helpers are forked using CLONE_FILES, they must resolve
shared fds to create their own service fds. This patch allows
that.

I've digged in the code, and there is no a reason, we need
pid_rst_prio() during choosing of fdt restorer. So, this
case may be safely deleted, which guarantees, that in case
of TASK_HELPER, the restorer of fdt will be parent, i.e.,
no one TASK_HELPER will be restorer of fdt.

v5: New
Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

criu/files.c

@@ -1302,6 +1302,7 @@ int prepare_fds(struct pstree_item *me)
 		}
 	}
 
+	BUG_ON(current->pid->state == TASK_HELPER);
 	ret = open_fdinfos(me);
 
 	if (rsti(me)->fdt)
@@ -1442,8 +1443,6 @@ int shared_fdt_prepare(struct pstree_item *item)
 	rsti(item)->fdt = fdt;
 	rsti(item)->service_fd_id = fdt->nr;
 	fdt->nr++;
-	if (pid_rst_prio(vpid(item), fdt->pid))
-		fdt->pid = vpid(item);
 
 	return 0;
 }