criu: tty: pick only master as a control terminal

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>

tty.c

@@ -1185,6 +1185,9 @@ int tty_setup_slavery(void)
 		    info->driver->type == TTY_TYPE__CTTY)
 			continue;
 
+		if (!tty_is_master(info))
+			continue;
+
 		info->ctl_tty = info;
 		pr_debug("ctl tty leader %x\n", info->tfe->id);
 		peer = info;