tty_find_restoring_task will prepare control terminal (by tcsetpgrp)
only if tty is master.
By a chance it's possible to pick slave terminal as a control, which
will refrain restoring control terminal.

https://jira.sw.ru/browse/PSBM-42093Signed-off-by: Dmitry Safonov <dsafonov@odin.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>