tty: Fix SIGTTOU not sent with tcflush()

Commit 'e7f3880cd9b98c5bf9391ae7acdec82b75403776'
tty: Fix recursive deadlock in tty_perform_flush()
introduced a regression where tcflush() does not generate
SIGTTOU for background process groups.

Make sure ioctl(TCFLSH) calls tty_check_change() when
invoked from the line discipline.

Cc: [e-mail] # v3.10+
Reported-by: Oleg Nesterov <[e-mail]>
Signed-off-by: Peter Hurley <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/tty_ioctl.c

···
 		}
 		return 0;
 	case TCFLSH:
+		retval = tty_check_change(tty);
+		if (retval)
+			return retval;
 		return __tty_perform_flush(tty, arg);
 	default:
 		/* Try the mode commands */