seize: set cr_errno in collect_pstree

Before cr_errno was called from open_pid_proc(), but
now it isn't called if a root task doesn't exist.

Reported-by: Mr Jenkins
Fixes: b9b0730c ("ptrace: split task_seize into seize_catch_task and seize_wait_task")
Signed-off-by: Andrew Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>