tty: Fix path inversion for /dev/pts/ptmx

Need to compare the /pts/ part in the
former path, otherwise it will produce
bogus /dev/pts/pts/index form.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>