pty: fix O_CLOEXEC for TIOCGPTPEER

It was being ignored because the flags were not passed to fd allocation.

Fixes: 54ebbfb1 ("tty: add TIOCGPTPEER ioctl")
Signed-off-by: Matthijs van Duin <matthijsvanduin@gmail.com>
Acked-by: Aleksa Sarai <asarai@suse.de>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>